Add RabbitMQ support for payment hooks (#80)

src/error.cpp

@@ -99,6 +99,8 @@ namespace lws
         return "An unknown in-process message was received";
       case error::system_clock_invalid_range:
         return "System clock is out of range for account storage format";
+      case error::rmq_failure:
+        return "Failure within the RMQ library";
       case error::tx_relay_failed:
         return "The daemon failed to relay transaction from REST client";
       default: