Improved ‘guix system’ error reporting

[Ludovic Courtès]

On IRC Christian Grothoff reported that ‘guix system init’ wouldn’t
provide much information beyond the error message.

Commit 2abcc97 improves that by showing (or trying to) the source code
location of the bottom-most stack frame.  This required getting the
user’s file auto-compiled, so we would get a meaningful stack frame
(when the file is evaluated, as was the case before, all we see is calls
to ‘eval’.)

Commit db03030 adds a ‘--on-error’ option to ‘guix system’ with the
possibility to start Guile’s debugger.

Feedback welcome!

Ludo’.