AMQP-319 Don't Suppress Listener Stack Traces

Currently, fatal listener startup exceptions only get
the full stack trace logged if logging at DEBUG level.

This makes diagnostics difficult in production.

Only suppress the full stack trace at WARN level if
the fatal exception is an `AmqpConnectException`.
Merge pull request #100 from garyrussell/AMQP-319

* garyrussell-AMQP-319:
  AMQP-319 Don't Suppress Listener Stack Traces