Return error message in AMQP response after failed conversion

When a problem occurs during the conversion from Core to AMQP, the code catches any exception that happens, logs it at the debug level, then returns a string value saying "Conversion to AMQP error!". It would be nice if the response string just includes the exception's message, and I've submitted a PR to that effect:

https://github.com/apache/activemq-artemis/pull/3060