Auto merge of #17483 - jyc:fail-properly, r=wafflespeanut
Auto merge of #17483 - jyc:fail-properly, r=wafflespeanut

mach: Fail properly if we can't find the path for OSMesa or GLAPI.

Currently we check `if not (osmesa_path and glapi_path)` after using
`path.join`. It seems we actually want to check whether
`find_dep_path_newest` returns something, because if calls to that
function fail, we'll instead get an error from `path.join` about
`NoneType` not having attribute `endswith` (it expects a string), which
preempts this check.

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