Show error stack trace when simple descriptor handler fails to parse a descriptor

Currently, the error stack trace is shown in gateway.log only, if the org.apache.knox.gateway log level is set to DEBUG:

  @Message( level = MessageLevel.ERROR, text = "An error occurred while processing {0} : {1}" )
  void simpleDescriptorHandlingError(String simpleDesc,
                                     @StackTrace(level = MessageLevel.DEBUG) Exception e);

This makes our lives hard when dealing with errors related to events coming from CM configuration monitoring.
I recommend showing this information even on INFO level.