Review HL7v2 exception aggregation when generating NAKs

[unixoid]

In org.openehealth.ipf.modules.hl7.CompositeHL7v2Exception#populateMessage() we have to create a cumulative HL7v2 NAK from multiple exceptions. In the current implementation, a lot of information goes lost, because we try to put all possible data in a single ERR segment and not all fields there are repeatable --- for example, there can be only one numerical error code. A possible solution would be to use multiple ERR segments, when the message structure allowsus to have them.