Avoid hardcoded references to rustc in download()
Avoid hardcoded references to rustc in download()

Use the `desc` parameter instead to make the error messages
customized for the actual download.

Also use new-style format strings.

anthgur/servo,splav/servo,cbrewster/servo,canaltinova/servo,avadacatavra/servo,dati91/servo,DominoTree/servo,larsbergstrom/servo,upsuper/servo,nnethercote/servo,nnethercote/servo,jimberlage/servo,mbrubeck/servo,canaltinova/servo,avadacatavra/servo,paulrouget/servo,KiChjang/servo,jimberlage/servo,cbrewster/servo,dsandeephegde/servo,larsbergstrom/servo,mbrubeck/servo,rnestler/servo,dsandeephegde/servo,peterjoel/servo,peterjoel/servo,pyfisch/servo,saneyuki/servo,szeged/servo,SimonSapin/servo,CJ8664/servo,mattnenterprise/servo,nnethercote/servo,fiji-flo/servo,anthgur/servo,pyfisch/servo,cbrewster/servo,dsandeephegde/servo,jimberlage/servo,DominoTree/servo,paulrouget/servo,rnestler/servo,splav/servo,mattnenterprise/servo,splav/servo,fiji-flo/servo,dsandeephegde/servo,anthgur/servo,larsbergstrom/servo,sadmansk/servo,splav/servo,thiagopnts/servo,sadmansk/servo,cbrewster/servo,thiagopnts/servo,sadmansk/servo,dati91/servo,KiChjang/servo,pyfisch/servo,cbrewster/servo,avadacatavra/servo,dsandeephegde/servo,dati91/servo,SimonSapin/servo,anthgur/servo,DominoTree/servo,DominoTree/servo,DominoTree/servo,KiChjang/servo,splav/servo,mattnenterprise/servo,cbrewster/servo,larsbergstrom/servo,jimberlage/servo,avadacatavra/servo,ConnorGBrewster/servo,mbrubeck/servo,thiagopnts/servo,splav/servo,mattnenterprise/servo,ConnorGBrewster/servo,szeged/servo,ConnorGBrewster/servo,mattnenterprise/servo,notriddle/servo,thiagopnts/servo,thiagopnts/servo,ConnorGBrewster/servo,dati91/servo,fiji-flo/servo,saneyuki/servo,upsuper/servo,paulrouget/servo,upsuper/servo,CJ8664/servo,saneyuki/servo,paulrouget/servo,dsandeephegde/servo,dati91/servo,notriddle/servo,avadacatavra/servo,rnestler/servo,mattnenterprise/servo,nnethercote/servo,upsuper/servo,eddyb/servo,pyfisch/servo,sadmansk/servo,szeged/servo,ConnorGBrewster/servo,SimonSapin/servo,mattnenterprise/servo,saneyuki/servo,mbrubeck/servo,saneyuki/servo,SimonSapin/servo,dati91/servo,szeged/servo,szeged/servo,peterjoel/servo,jimberlage/servo,paulrouget/servo,eddyb/servo,canaltinova/servo,szeged/servo,peterjoel/servo,fiji-flo/servo,sadmansk/servo,CJ8664/servo,canaltinova/servo,peterjoel/servo,anthgur/servo,nnethercote/servo,KiChjang/servo,szeged/servo,mattnenterprise/servo,notriddle/servo,upsuper/servo,CJ8664/servo,KiChjang/servo,notriddle/servo,peterjoel/servo,CJ8664/servo,pyfisch/servo,emilio/servo,emilio/servo,szeged/servo,paulrouget/servo,sadmansk/servo,rnestler/servo,fiji-flo/servo,SimonSapin/servo,saneyuki/servo,canaltinova/servo,saneyuki/servo,thiagopnts/servo,larsbergstrom/servo,szeged/servo,eddyb/servo,larsbergstrom/servo,saneyuki/servo,avadacatavra/servo,DominoTree/servo,KiChjang/servo,splav/servo,pyfisch/servo,fiji-flo/servo,sadmansk/servo,paulrouget/servo,mbrubeck/servo,larsbergstrom/servo,upsuper/servo,KiChjang/servo,SimonSapin/servo,rnestler/servo,splav/servo,nnethercote/servo,emilio/servo,avadacatavra/servo,eddyb/servo,emilio/servo,pyfisch/servo,saneyuki/servo,CJ8664/servo,pyfisch/servo,anthgur/servo,CJ8664/servo,fiji-flo/servo,cbrewster/servo,mbrubeck/servo,larsbergstrom/servo,dsandeephegde/servo,nnethercote/servo,DominoTree/servo,emilio/servo,canaltinova/servo,peterjoel/servo,emilio/servo,fiji-flo/servo,splav/servo,sadmansk/servo,canaltinova/servo,paulrouget/servo,saneyuki/servo,thiagopnts/servo,ConnorGBrewster/servo,eddyb/servo,KiChjang/servo,notriddle/servo,larsbergstrom/servo,jimberlage/servo,peterjoel/servo,nnethercote/servo,notriddle/servo,ConnorGBrewster/servo,peterjoel/servo,notriddle/servo,pyfisch/servo,DominoTree/servo,upsuper/servo,KiChjang/servo,eddyb/servo,rnestler/servo,DominoTree/servo,thiagopnts/servo,rnestler/servo,emilio/servo,upsuper/servo,splav/servo,jimberlage/servo,emilio/servo,paulrouget/servo,eddyb/servo,dati91/servo,DominoTree/servo,eddyb/servo,ConnorGBrewster/servo,jimberlage/servo,SimonSapin/servo,KiChjang/servo,pyfisch/servo,nnethercote/servo,szeged/servo,canaltinova/servo,paulrouget/servo,rnestler/servo,emilio/servo,SimonSapin/servo,cbrewster/servo,mbrubeck/servo,dati91/servo,notriddle/servo,larsbergstrom/servo,notriddle/servo,CJ8664/servo,notriddle/servo,anthgur/servo,mbrubeck/servo,emilio/servo,anthgur/servo,jimberlage/servo,nnethercote/servo,peterjoel/servo,avadacatavra/servo,dsandeephegde/servo