More descriptive error when package constructor fails.
More descriptive error when package constructor fails.

- helps package_sanity test identify which package failed.
- encountered while upgrading versions in Adam's packages to the new format.

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