Auto merge of #11067 - autrilla:mach-errors, r=mbrubeck
Auto merge of #11067 - autrilla:mach-errors, r=mbrubeck

Mach now shows stderr when a virtualenv or pip call fails (fixes #11055)

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