When closing a session as a result of a problem, send a <stream:error> stanza.
When a session is closed because of some kind of problem (for instance: non-valid XML was received), that session is closed. Openfire should send a stream:error stanza right before the session gets closed.
In case of a well-formedness error detected by the XmlLightWeightParser, the code now triggers an "xml_not_well_formed" stream error. Other exceptions that cause the session to be closed will trigger a 'internal_server_error'.