cc: make error messages more clear
cc: make error messages more clear

- [x] Our wrapper error messages are sometimes hard to differentiate from other build
      output, so prefix all errors from `die()` with '[spack cc] ERROR:'

- [x] The error we raise when running, say, `fc` without a Fortran compiler was not
      clear enough. Clarify the message and the comment.

LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack