bug#23306: 'guix environment --container' silently rejects invalid --expose parameters

[Ludovic Courtès]

Passing a nonexistent file as --expose leads ‘guix environment’ to
silently fail:

--8<---------------cut here---------------start------------->8---
$ guix environment --ad-hoc guile --expose=does-not-exist --container
guix environment: warning: ambiguous package specification `guile'
guix environment: warning: choosing guile-2.0.11 from 
gnu/packages/guile.scm:125:2
$ echo $?
1
--8<---------------cut here---------------end--------------->8---

It would be nice to print an error message.

Ludo’.