Reconnect to IRC on send error

If any error is encountered while sending to IRC, try to reconnect.

This is an attempt to deal with this exception:

2013-10-07 23:00:00,937 ERROR gerritbot: Exception encountered in event loop
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/gerritbot/bot.py", line 225, in run
    self._read(event)
  File "/usr/local/lib/python2.7/dist-packages/gerritbot/bot.py", line 216, in _read
    self.change_merged(channel, data)
  File "/usr/local/lib/python2.7/dist-packages/gerritbot/bot.py", line 194, in change_merged
    self.ircbot.send(channel, msg)
  File "/usr/local/lib/python2.7/dist-packages/gerritbot/bot.py", line 98, in send
    self.connection.privmsg(channel, msg)
  File "/usr/local/lib/python2.7/dist-packages/irc/client.py", line 933, in privmsg
    self.send_raw("PRIVMSG %s :%s" % (target, text))
  File "/usr/local/lib/python2.7/dist-packages/irc/client.py", line 960, in send_raw
    sender = self.ssl.write if self.ssl else self.socket.send
AttributeError: 'NoneType' object has no attribute 'send'

Change-Id: I8c845626260ec7b9ee7f9e409adba4796dd367ee