Improved package.py error handling. (#2187)

- Detailed debug information is now handed back to the parent process
  from builds, for *any* type of exception.

- previously this only worked for Spack ProcessErrors, but now it works
  for any type of error raised in a child.

- Spack will print an error message and source code context for build
  errors by default.

- It will print a stack trace when using `spack -d`, even when the error
  occurred in the child process.