Improved error messages for compiler_for_spec when either zero or multiple compilers returned.
Improved error messages for compiler_for_spec when either zero or multiple compilers returned.

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