SPACK-18: simpler build error messages
SPACK-18: simpler build error messages

Suppress python stack trace on build error UNLESS in debug mode (spack -d).
Now spack shows errors with a single red arrow, and it's easier to find the actual build output.

skosukhin/spack,iulian787/spack,EmreAtes/spack,krafczyk/spack,iulian787/spack,tmerrick1/spack,mfherbst/spack,tmerrick1/spack,lgarren/spack,lgarren/spack,matthiasdiener/spack,tmerrick1/spack,TheTimmy/spack,LLNL/spack,iulian787/spack,TheTimmy/spack,TheTimmy/spack,mfherbst/spack,LLNL/spack,LLNL/spack,tmerrick1/spack,matthiasdiener/spack,mfherbst/spack,matthiasdiener/spack,skosukhin/spack,LLNL/spack,mfherbst/spack,skosukhin/spack,TheTimmy/spack,matthiasdiener/spack,lgarren/spack,EmreAtes/spack,krafczyk/spack,krafczyk/spack,EmreAtes/spack,mfherbst/spack,skosukhin/spack,matthiasdiener/spack,LLNL/spack,skosukhin/spack,tmerrick1/spack,iulian787/spack,EmreAtes/spack,EmreAtes/spack,krafczyk/spack,lgarren/spack,krafczyk/spack,iulian787/spack,TheTimmy/spack,lgarren/spack