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 7838cae3bc5f23c271c8baba0630d6969d8f1b55

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