Monitor: print message before quit for no array to monitor

Monitor: print message before quit for no array to monitor

If there is no array device to monitor, Monitor() will stop monitoring
at line 261 from the following code block,
 257                 if (!new_found) {
 258                         if (oneshot)
 259                                 break;
 260                         else if (!anyredundant) {
 261                                 break;
 262                         }

This change was introduced by commit 007087d0898a ("Monitor: stop
notifing about containers"). Before this commit, Monitor() will continue
and won't quit even there is no array to monitor.

It is fine to quit without any array device to monitor, but users may
wonder whether there is something wrong with mdadm program or their
configuration to make mdadm quit monitoring.

This patch adds a simple error message to indicate Monitor() quits for
array device to monitor, which makes users have hint to understand why
mdadm stops monitoring.

Reported-by: George Gkioulis <ggkioulis@suse.com>
Suggested-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>
Signed-off-by: Coly Li <colyli@suse.de>
Cc: Jes Sorensen <jsorensen@fb.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>