Irc: warn when applyModes target doesn't exist

This can happen when a network disconnects while spawnClient is setting modes on a newly spawned client, for example...

classes.py

@@ -644,10 +644,13 @@ class Irc():
                 else:
                     modelist.discard(real_mode)
         log.debug('(%s) Final modelist: %s', self.name, modelist)
+        try:
             if usermodes:
                 self.users[target].modes = modelist
             else:
                 self.channels[target].modes = modelist
+        except KeyError:
+            log.warning("(%s) Invalid MODE target %s (usermodes=%s)", self.name, target, usermodes)
 
     @staticmethod
     def _flip(mode):