auto merge of #2341 : Manishearth/servo/error-parse, r=jdm
auto merge of #2341 : Manishearth/servo/error-parse, r=jdm

Sometimes it's useful to bubble out the error from the URL parsing so the caller can deal with the result as per its needs.

[The XHR `open()`](http://xhr.spec.whatwg.org/#the-open()-method), for example, is one place where the bubbling out of an error is required.

Blocks #2282

WriterOfAlicrow/servo,pyfisch/servo,CJ8664/servo,mdibaiee/servo,Adenilson/prototype-viewing-distance,rixrix/servo,huonw/servo,tafia/servo,mukilan/servo,nnethercote/servo,pgonda/servo,CJ8664/servo,dvberkel/servo,bfrohs/servo,caldwell/servo,saneyuki/servo,emilio/servo,dhananjay92/servo,KiChjang/servo,splav/servo,emilio/servo,sadmansk/servo,chotchki/servo,larsbergstrom/servo,KiChjang/servo,kindersung/servo,rnestler/servo,szeged/servo,paulrouget/servo,shrenikgala/servo,A-deLuna/servo,akosel/servo,zhangjunlei26/servo,hyowon/servo,emilio/servo,deokjinkim/servo,ConnorGBrewster/servo,dvberkel/servo,pyfisch/servo,saneyuki/servo,nick-thompson/servo,zhangjunlei26/servo,GreenRecycleBin/servo,evilpie/servo,mbrubeck/servo,mbrubeck/servo,saratang/servo,AnthonyBroadCrawford/servo,szeged/servo,notriddle/servo,juzer10/servo,walac/servo,zentner-kyle/servo,zhangjunlei26/servo,snf/servo,notriddle/servo,akosel/servo,larsbergstrom/servo,ConnorGBrewster/servo,tschneidereit/servo,paulrouget/servo,rixrix/servo,srbhklkrn/SERVOENGINE,dhananjay92/servo,jlegendary/servo,s142857/servo,nick-thompson/servo,dati91/servo,walac/servo,RenaudParis/servo,indykish/servo,mt2d2/servo,pyfisch/servo,cbrewster/servo,luniv/servo,upsuper/servo,notriddle/servo,avadacatavra/servo,GyrosOfWar/servo,s142857/servo,nick-thompson/servo,notriddle/servo,RenaudParis/servo,fiji-flo/servo,rnestler/servo,zhangjunlei26/servo,thiagopnts/servo,caldwell/servo,codemac/servo,steveklabnik/servo,pyfisch/servo,Adenilson/prototype-viewing-distance,CJ8664/servo,michaelwu/servo,indykish/servo,aweinstock314/servo,rnestler/servo,WriterOfAlicrow/servo,indykish/servo,saneyuki/servo,walac/servo,mt2d2/servo,kindersung/servo,ruud-v-a/servo,evilpie/servo,pyecs/servo,saneyuki/servo,jlegendary/servo,seanmonstar/servo,SimonSapin/servo,CJ8664/servo,indykish/servo,jdramani/servo,dhananjay92/servo,avadacatavra/servo,nrc/servo,dagnir/servo,dagnir/servo,mattnenterprise/servo,walac/servo,caldwell/servo,g-k/servo,dati91/servo,wartman4404/servo,aidanhs/servo,rentongzhang/servo,brendandahl/servo,mt2d2/servo,eddyb/servo,boghison/servo,luniv/servo,GreenRecycleBin/servo,j3parker/servo,dmarcos/servo,tempbottle/servo,bjwbell/servo,tschneidereit/servo,meh/servo,mbrubeck/servo,mt2d2/servo,jgraham/servo,jgraham/servo,DominoTree/servo,avadacatavra/servo,rixrix/servo,dvberkel/servo,akosel/servo,jlegendary/servo,michaelwu/servo,KiChjang/servo,rentongzhang/servo,youprofit/servo,mdibaiee/servo,snf/servo,samfoo/servo,nerith/servo,notriddle/servo,aweinstock314/servo,saneyuki/servo,eddyb/servo,shrenikgala/servo,codemac/servo,upsuper/servo,jlegendary/servo,dsandeephegde/servo,GreenRecycleBin/servo,vks/servo,brendandahl/servo,GreenRecycleBin/servo,kindersung/servo,A-deLuna/servo,KiChjang/servo,rixrix/servo,szeged/servo,paulrouget/servo,larsbergstrom/servo,samfoo/servo,nnethercote/servo,rentongzhang/servo,srbhklkrn/SERVOENGINE,akosel/servo,srbhklkrn/SERVOENGINE,ConnorGBrewster/servo,rixrix/servo,pgonda/servo,Adenilson/prototype-viewing-distance,evilpie/servo,dhananjay92/servo,eddyb/servo,huonw/servo,ryancanhelpyou/servo,echochamber/servo,nnethercote/servo,DominoTree/servo,notriddle/servo,notriddle/servo,aidanhs/servo,larsbergstrom/servo,thiagopnts/servo,bfrohs/servo,aweinstock314/servo,pgonda/servo,jdramani/servo,canaltinova/servo,wartman4404/servo,peterjoel/servo,zentner-kyle/servo,emilio/servo,nnethercote/servo,canaltinova/servo,srbhklkrn/SERVOENGINE,runarberg/servo,runarberg/servo,luniv/servo,karlito40/servo,eddyb/servo,dagnir/servo,fiji-flo/servo,upsuper/servo,GyrosOfWar/servo,tafia/servo,szeged/servo,kindersung/servo,rixrix/servo,wartman4404/servo,rentongzhang/servo,saratang/servo,juzer10/servo,rnestler/servo,shrenikgala/servo,ruud-v-a/servo,echochamber/servo,SimonSapin/servo,kindersung/servo,nick-thompson/servo,GreenRecycleBin/servo,saratang/servo,g-k/servo,jdramani/servo,SimonSapin/servo,tempbottle/servo,mbrubeck/servo,seanmonstar/servo,bjwbell/servo,wpgallih/servo,AnthonyBroadCrawford/servo,aweinstock314/servo,thiagopnts/servo,seanmonstar/servo,dati91/servo,codemac/servo,paulrouget/servo,mbrubeck/servo,zhangjunlei26/servo,dmarcos/servo,nrc/servo,dsandeephegde/servo,j3parker/servo,runarberg/servo,echochamber/servo,tempbottle/servo,thiagopnts/servo,j3parker/servo,Adenilson/prototype-viewing-distance,s142857/servo,Adenilson/prototype-viewing-distance,jdramani/servo,kindersung/servo,pyfisch/servo,walac/servo,juzer10/servo,szeged/servo,boghison/servo,pyfisch/servo,youprofit/servo,karlito40/servo,A-deLuna/servo,saratang/servo,seanmonstar/servo,GyrosOfWar/servo,zentner-kyle/servo,WriterOfAlicrow/servo,peterjoel/servo,zentner-kyle/servo,fiji-flo/servo,mdibaiee/servo,KiChjang/servo,nick-thompson/servo,evilpie/servo,CJ8664/servo,anthgur/servo,j3parker/servo,dagnir/servo,vks/servo,youprofit/servo,paulrouget/servo,tschneidereit/servo,steveklabnik/servo,fiji-flo/servo,jgraham/servo,AnthonyBroadCrawford/servo,jlegendary/servo,jgraham/servo,pyecs/servo,saneyuki/servo,srbhklkrn/SERVOENGINE,froydnj/servo,s142857/servo,ruud-v-a/servo,emilio/servo,zhangjunlei26/servo,upsuper/servo,g-k/servo,evilpie/servo,g-k/servo,indykish/servo,larsbergstrom/servo,paulrouget/servo,emilio/servo,nerith/servo,sadmansk/servo,vks/servo,dvberkel/servo,brendandahl/servo,hyowon/servo,hyowon/servo,runarberg/servo,juzer10/servo,sadmansk/servo,ryancanhelpyou/servo,jimberlage/servo,ryancanhelpyou/servo,ryancanhelpyou/servo,pyecs/servo,zentner-kyle/servo,srbhklkrn/SERVOENGINE,hyowon/servo,splav/servo,evilpie/servo,Shraddha512/servo,avadacatavra/servo,A-deLuna/servo,jimberlage/servo,shrenikgala/servo,nnethercote/servo,indykish/servo,larsbergstrom/servo,larsbergstrom/servo,dati91/servo,nnethercote/servo,jimberlage/servo,peterjoel/servo,deokjinkim/servo,youprofit/servo,tempbottle/servo,splav/servo,zhangjunlei26/servo,aidanhs/servo,thiagopnts/servo,rentongzhang/servo,chotchki/servo,peterjoel/servo,upsuper/servo,nrc/servo,dmarcos/servo,dsandeephegde/servo,tschneidereit/servo,RenaudParis/servo,bjwbell/servo,huonw/servo,dsandeephegde/servo,ryancanhelpyou/servo,A-deLuna/servo,Adenilson/prototype-viewing-distance,ruud-v-a/servo,indykish/servo,fiji-flo/servo,dsandeephegde/servo,snf/servo,jgraham/servo,boghison/servo,nrc/servo,wartman4404/servo,seanmonstar/servo,splav/servo,brendandahl/servo,anthgur/servo,deokjinkim/servo,youprofit/servo,hyowon/servo,AnthonyBroadCrawford/servo,steveklabnik/servo,mdibaiee/servo,s142857/servo,KiChjang/servo,thiagopnts/servo,larsbergstrom/servo,jdramani/servo,rentongzhang/servo,CJ8664/servo,canaltinova/servo,eddyb/servo,dmarcos/servo,eddyb/servo,canaltinova/servo,tafia/servo,meh/servo,sadmansk/servo,dati91/servo,canaltinova/servo,pyfisch/servo,kindersung/servo,emilio/servo,mattnenterprise/servo,rixrix/servo,snf/servo,sadmansk/servo,chotchki/servo,fiji-flo/servo,pgonda/servo,saratang/servo,rnestler/servo,tschneidereit/servo,tafia/servo,rixrix/servo,saneyuki/servo,snf/servo,avadacatavra/servo,mbrubeck/servo,jgraham/servo,samfoo/servo,codemac/servo,tschneidereit/servo,nerith/servo,anthgur/servo,snf/servo,anthgur/servo,GreenRecycleBin/servo,pgonda/servo,karlito40/servo,mukilan/servo,KiChjang/servo,CJ8664/servo,chotchki/servo,DominoTree/servo,cbrewster/servo,chotchki/servo,mattnenterprise/servo,steveklabnik/servo,mattnenterprise/servo,Adenilson/prototype-viewing-distance,s142857/servo,pyecs/servo,samfoo/servo,mdibaiee/servo,Shraddha512/servo,nnethercote/servo,WriterOfAlicrow/servo,canaltinova/servo,SimonSapin/servo,DominoTree/servo,tempbottle/servo,mukilan/servo,paulrouget/servo,jimberlage/servo,codemac/servo,nerith/servo,Shraddha512/servo,GreenRecycleBin/servo,samfoo/servo,splav/servo,ConnorGBrewster/servo,larsbergstrom/servo,GreenRecycleBin/servo,tafia/servo,echochamber/servo,dvberkel/servo,michaelwu/servo,codemac/servo,peterjoel/servo,froydnj/servo,mt2d2/servo,RenaudParis/servo,dati91/servo,g-k/servo,ConnorGBrewster/servo,jlegendary/servo,aidanhs/servo,indykish/servo,sadmansk/servo,WriterOfAlicrow/servo,shrenikgala/servo,anthgur/servo,nnethercote/servo,evilpie/servo,michaelwu/servo,eddyb/servo,zentner-kyle/servo,Shraddha512/servo,SimonSapin/servo,Shraddha512/servo,codemac/servo,pgonda/servo,KiChjang/servo,juzer10/servo,akosel/servo,ConnorGBrewster/servo,caldwell/servo,dsandeephegde/servo,jimberlage/servo,wpgallih/servo,nerith/servo,splav/servo,peterjoel/servo,notriddle/servo,bjwbell/servo,szeged/servo,froydnj/servo,DominoTree/servo,karlito40/servo,Shraddha512/servo,RenaudParis/servo,nerith/servo,deokjinkim/servo,tempbottle/servo,g-k/servo,luniv/servo,DominoTree/servo,michaelwu/servo,mattnenterprise/servo,ryancanhelpyou/servo,saratang/servo,nrc/servo,g-k/servo,michaelwu/servo,pyecs/servo,hyowon/servo,GreenRecycleBin/servo,brendandahl/servo,jimberlage/servo,jimberlage/servo,cbrewster/servo,mt2d2/servo,paulrouget/servo,bfrohs/servo,karlito40/servo,wpgallih/servo,seanmonstar/servo,nrc/servo,huonw/servo,wartman4404/servo,bfrohs/servo,echochamber/servo,fiji-flo/servo,rnestler/servo,walac/servo,froydnj/servo,meh/servo,dati91/servo,walac/servo,AnthonyBroadCrawford/servo,splav/servo,wpgallih/servo,dagnir/servo,vks/servo,evilpie/servo,upsuper/servo,RenaudParis/servo,AnthonyBroadCrawford/servo,mattnenterprise/servo,jgraham/servo,deokjinkim/servo,emilio/servo,huonw/servo,anthgur/servo,karlito40/servo,evilpie/servo,froydnj/servo,wpgallih/servo,meh/servo,aidanhs/servo,rnestler/servo,DominoTree/servo,runarberg/servo,mattnenterprise/servo,pyfisch/servo,j3parker/servo,echochamber/servo,shrenikgala/servo,wpgallih/servo,upsuper/servo,wartman4404/servo,froydnj/servo,meh/servo,juzer10/servo,pyecs/servo,cbrewster/servo,KiChjang/servo,KiChjang/servo,karlito40/servo,canaltinova/servo,rixrix/servo,runarberg/servo,pgonda/servo,caldwell/servo,avadacatavra/servo,hyowon/servo,rnestler/servo,RenaudParis/servo,mdibaiee/servo,pyecs/servo,steveklabnik/servo,dvberkel/servo,bjwbell/servo,DominoTree/servo,dhananjay92/servo,splav/servo,DominoTree/servo,bfrohs/servo,vks/servo,indykish/servo,sadmansk/servo,deokjinkim/servo,saneyuki/servo,splav/servo,nick-thompson/servo,szeged/servo,pyfisch/servo,brendandahl/servo,aidanhs/servo,wpgallih/servo,meh/servo,luniv/servo,s142857/servo,mt2d2/servo,avadacatavra/servo,GyrosOfWar/servo,boghison/servo,WriterOfAlicrow/servo,boghison/servo,luniv/servo,j3parker/servo,tafia/servo,szeged/servo,szeged/servo,akosel/servo,SimonSapin/servo,nrc/servo,szeged/servo,caldwell/servo,bjwbell/servo,huonw/servo,aweinstock314/servo,A-deLuna/servo,dhananjay92/servo,nnethercote/servo,sadmansk/servo,larsbergstrom/servo,juzer10/servo,fiji-flo/servo,thiagopnts/servo,chotchki/servo,anthgur/servo,meh/servo,anthgur/servo,boghison/servo,jimberlage/servo,ConnorGBrewster/servo,notriddle/servo,dmarcos/servo,akosel/servo,pyfisch/servo,zentner-kyle/servo,luniv/servo,mukilan/servo,samfoo/servo,dsandeephegde/servo,emilio/servo,wartman4404/servo,CJ8664/servo,dmarcos/servo,jdramani/servo,paulrouget/servo,srbhklkrn/SERVOENGINE,dvberkel/servo,notriddle/servo,rentongzhang/servo,peterjoel/servo,peterjoel/servo,vks/servo,wpgallih/servo,tafia/servo,huonw/servo,cbrewster/servo,peterjoel/servo,thiagopnts/servo,jimberlage/servo,vks/servo,samfoo/servo,aweinstock314/servo,emilio/servo,peterjoel/servo,saneyuki/servo,mukilan/servo,dhananjay92/servo,GyrosOfWar/servo,aweinstock314/servo,Shraddha512/servo,nnethercote/servo,echochamber/servo,wpgallih/servo,ruud-v-a/servo,ConnorGBrewster/servo,dagnir/servo,nerith/servo,splav/servo,zhangjunlei26/servo,nick-thompson/servo,tempbottle/servo,saneyuki/servo,SimonSapin/servo,mukilan/servo,aidanhs/servo,youprofit/servo,dmarcos/servo,ruud-v-a/servo,GyrosOfWar/servo,dagnir/servo,SimonSapin/servo,mbrubeck/servo,canaltinova/servo,bjwbell/servo,AnthonyBroadCrawford/servo,j3parker/servo,jdramani/servo,caldwell/servo,runarberg/servo,tschneidereit/servo,eddyb/servo,dati91/servo,upsuper/servo,ryancanhelpyou/servo,bfrohs/servo,snf/servo,deokjinkim/servo,paulrouget/servo,cbrewster/servo,youprofit/servo,zhangjunlei26/servo,mattnenterprise/servo,avadacatavra/servo,A-deLuna/servo,dsandeephegde/servo,WriterOfAlicrow/servo,mdibaiee/servo,shrenikgala/servo,steveklabnik/servo,jlegendary/servo,brendandahl/servo,boghison/servo,GyrosOfWar/servo,saratang/servo,cbrewster/servo,steveklabnik/servo,cbrewster/servo,mbrubeck/servo,DominoTree/servo