Improve exception type and error message when connection to XMPP server fails

[amotl]

Problem

When connecting to an XMPP server fails, the library raises a cryptic exception that does not quite reveal what's going on.

DEBUG: socket       start Plugging <xmpp.transports.TCPsocket object at 0x105ae3620> into <xmpp.client.Client object at 0x105ae3380>
DEBUG: socket       warn  An error occurred while looking up _xmpp-client._tcp.localhost
DEBUG: socket       error Failed to connect to remote host ('localhost', 5222): Connection refused (61)
DEBUG: socket       stop  Plugging <xmpp.transports.TCPsocket object at 0x105ae3620> out of <xmpp.client.Client object at 0x105ae3380>.
Traceback (most recent call last):
  File "/path/to/xmpp-message", line 10, in <module>
    sys.exit(simple_message())
             ~~~~~~~~~~~~~~^^
  File "/path/to/xmpppy/xmpp/cli.py", line 74, in simple_message
    send_message(options.jabberid, options.password, options.receiver, options.message, debug=options.debug)
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/path/to/xmpppy/xmpp/cli.py", line 27, in send_message
    retval = connection.auth(user=jid.getNode(), password=password, resource=jid.getResource())
  File "/path/to/xmpppy/xmpp/client.py", line 217, in auth
    while not self.Dispatcher.Stream._document_attrs and self.Process(1): pass
              ^^^^^^^^^^^^^^^
AttributeError: 'Client' object has no attribute 'Dispatcher'