XDS error handling: accumulation of errors

[unixoid]

XDS validators should not throw exception after the very first found error, but try to discover all other errors as well (when possible), and report them all to the user in a single "package". In other words, they should follow the same approach as HL7v2 validators, whose execution result is a list of error messages, from which an org.openehealth.ipf.modules.hl7.CompositeHL7v2Exception instance is then created.

This will be very helpful when it should be discovered why a particular message is not accepted by an XDS endpoint --- it will not be neccessary to resend the message multiple times.