vcl: CUPS printing: notify user if spooling failed

CUPS does not accept jobs for a printer that has been stopped; pop up
an error message in this case instead of silently ignoring failure.

Change-Id: I1bc9120c9c9f438e350c964ed7ef29924a72ce78
(cherry picked from commit cdb405f4e4b168d1e2f6dea9ac985df319a76015)
Reviewed-on: https://gerrit.libreoffice.org/8115
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>