report error if failed process captures stderr (#9293)
report error if failed process captures stderr (#9293)

When a Spack Executable was configured to capture stderr and the
process failed, the error messages of the process were discarded.
This made it difficult to understand why the process failed. The
exception is now updated to include the stderr of the process when
the Executable captures stderr.
iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack