Collective cvmfs_server operations should return non-zero on failure

The cvmfs_server gc -a and cvmfs_server check -a return zero even if the operation fails for one or multiple of the repositories.  This should be changed to properly indicate success or failure through the return code.

This has an impact on existing cron jobs, which admins may want to change to cvmfs_server gc -a || true to restore the current behavior.