Improve output of 'server migrate --wait'

We have seen users interpret the current output ('Complete') of
'server migrate --wait' as if it meant success as well and be surprised
when later they learned that the migration was complete and failed.

This change adds a pointer to the that output, how to actually check the
success/failure of a migration, hoping to eliminate this user confusion.

Change-Id: I09030705a39405366d6202a5ac743cc4d1ddd63c

openstackclient/compute/v2/server.py

@@ -3211,7 +3211,12 @@ revert to release the new server and restart the old one."""
                 success_status=['active', 'verify_resize'],
                 callback=_show_progress,
             ):
-                self.app.stdout.write(_('Complete\n'))
+                self.app.stdout.write(
+                    _(
+                        'Complete, check success/failure by '
+                        'openstack server migration/event list/show\n'
+                    )
+                )
             else:
                 msg = _('Error migrating server: %s') % server.id
                 raise exceptions.CommandError(msg)