gfs2: Issue message when revokes cannot be written

Before this patch, function gfs2_ail_empty_gl would silently return an
error to the caller. This would get silently set into sd_log_error which
would cause a withdraw, but there was no indication why the file system
was withdrawn. This patch adds a fs_err to log the appropriate error
message.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>