callbacks: Properly handle nested command errors

Using Raise=True was only papering over the real problem in the nested
command error handling.  The actual issue is that we were trying to
return an IrcMsg from NestedCommandsIrcProxy.__init__.  Dropping
Raise=True and moving return to its own line is the correct fix and
resolves the test failure in testMaximumNestingDepth.

This commit reverts 7838cae3bc

Signed-off-by: James McCoy <vega.james@gmail.com>

src/callbacks.py

@@ -587,8 +587,9 @@ class NestedCommandsIrcProxy(ReplyIrcProxy):
         if maxNesting and self.nested > maxNesting:
             log.warning('%s attempted more than %s levels of nesting.',
                         self.msg.prefix, maxNesting)
-            return self.error('You\'ve attempted more nesting than is '
-                              'currently allowed on this bot.', Raise=True)
+            self.error('You\'ve attempted more nesting than is '
+                       'currently allowed on this bot.')
+            return
         # The deepcopy here is necessary for Scheduler; it re-runs already
         # tokenized commands.  There's a possibility a simple copy[:] would
         # work, but we're being careful.