fix error message when not connecting to AMQP

New log:

    2014-09-08 17:51:25+0200 [Uninitialized] 'AmqpFactory connection failed (Connection was refused by other side: 111: Connection refused.)'
    2014-09-08 17:51:25+0200 [Uninitialized] <twisted.internet.tcp.Connector instance at 0x7f42cf47ab90> will retry in 3 seconds

old log:

    2014-09-08 17:58:40+0200 [Uninitialized] "Connection failed: <twisted.python.failure.Failure <class 'twisted.internet.error.ConnectionRefusedError'>>"
    2014-09-08 17:58:40+0200 [Uninitialized] <twisted.internet.tcp.Connector instance at 0x7f906efefb90> will retry in 2 seconds
    2014-09-08 17:58:40+0200 [Uninitialized] Stopping factory <vumi.service.AmqpFactory instance at 0x7f906cac86c8>