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If the message objects returned from the back end have both the TYPE and MESSAGE fields, then setting messageKeys is all the configuration you need to do (these are automatically dealt with)you do not need to modify the method convertRFCMessage. Error messages are automatically handled by mortar. However, if some other type of object is returned instead that doesn't have these fields, you will need to add code in the convertRFCMessage method in your service implementation to convert these message objects to the RFCMessage type. Otherwise, the method convertRFCMessage should return null.
In 99.9% of the cases this method is left alone: 