Response when API cannot be accessed should be improved

When the API implementation is not available, the current response from the gateway is a 500 status code, the message "io.apiman.gateway.engine.beans.exceptions.ConnectionException: Not connected.", and a stack trace (in releases before 1.3.0, the message was null). This is not very useful to the caller, and it is not really an internal error. A more appropriate status code (maybe 502, Bad Gateway, or 503, Service Unavailable) should be used, with an appropriate message. The stack trace should be in the log for debugging purposes, not returned to the caller.