install: Gracefully handle corner cases with 'guix system init foo /'.

* gnu/build/install.scm (evaluate-populate-directive): Wrap body in
  "catch 'system-error", and report clear errors.  In the symlink case,
  retry up EEXIST.
  (populate-root-file-system): Remove /var/guix/profiles/system-1-link
  before attempting to create it.