This method parses the SWIFT message header and returns the mesage type related information. The return string contains the information useful for the user to identify the message.
Condition:
- the message has to be a MT message, a NACK or an ACK.
- For all messages with Service_Identifier == 01 (User and System Messages), Message_Type will contain the actual message type. Example: 541, 010 etc. - For all Service Messages, only Service_Identifier is populated. Example: Service_Identifier = 21 for ACK/NACK