Use a JMRI 403 error instead of an NSXMLParser 5 error

NSXMLParserPrematureDocumentEndError (code 5) is returned (most likely)
when the XmlIO server cannot create the object requested, so return a
403 error indicating that the requested object could not be created.
Fixes #18.
Explicitely set _requiredServices to nil.

This is not needed, but does more clearly indicate why we later test
that _requiredServices is not nil.