If you call GIMP with batch commands and ask it to quit immediately, you
are likely interested by failure information. For this reason, let's now
report exit code other than success, but only in such case. In
particular, even if the batch commands fail, but GIMP is not set to exit
immediately, we continue reporting SUCCESS run at the very end (when
exiting interactively).
Note that I hardcode a few exit values, as standardly found on Linux
(but not using headers which may not be found on all platforms; in
particular, Windows apparently doesn't use any standard code other than
0 for success, from what my searches return).
Additionally, when several commands are requested, GIMP will now stop at
the first failing and will return its error code, and print a message on
stderr to easily report the failed command for easier debugging.
