Config error handling

[photm5]

If you forget to supply a nick in config.json, a stacktrace like this will
be shown:

  File "/home/shakmar/workspaces/python/pybot/pybot/irc.py", line 65, in run
    self._connect ()
  File "/home/shakmar/workspaces/python/pybot/pybot/irc.py", line 93, in _connect
    self._register ()
  File "/home/shakmar/workspaces/python/pybot/pybot/irc.py", line 101, in _register
    self.send_event ( nick_event )
  File "/home/shakmar/workspaces/python/pybot/pybot/irc.py", line 180, in send_event
    self._socket.send ( "%s\r\n" % event.signal )
  File "/home/shakmar/workspaces/python/pybot/pybot/irc.py", line 268, in signal
    if " " in arg:
TypeError: argument of type 'NoneType' is not iterable

A nicely formatted error message would be preferrable.