[Reviewer: Alex] Quiet routing errors

Fixes https://github.com/Metaswitch/cpp-common/issues/96.

Sample output:

06-05-2014 14:08:49.961 Error diameterstack.cpp:186: Routing error: 'No remaining suitable candidate to route the message to' for message with Command-Code 271, Destination-Host ec2-54-197-167-141.compute-1.amazonaws.com and Destination-Realm rkd.cw-ngv.com
06-05-2014 14:08:49.961 Warning peer_message_sender.cpp:125: Failed to send ACR to ec2-54-197-167-141.compute-1.amazonaws.com (number 0)
06-05-2014 14:08:49.961 Error peer_message_sender.cpp:145: Failed to connect to all CCFs, message not sent
06-05-2014 14:08:49.961 Warning session_manager.cpp:264: Session for abcd received error from CDF