Handle font load errors in FontContext
Handle font load errors in FontContext

Thread font load errors from platform font loading code to
FontContext::get_layout_font_group_for_style, and cache the failure
result, instead of panicing the thread when a font fails to load.

Before this patch, a failed font load would panic a LayoutTask,
causing a cascade of panics, eventually aborting the whole engine
during a panic within panic.

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