code: protected String reportValidationProblem(InputProblemReporter rep, String msg)
          throws XMLStreamException
      {
          rep.reportValidationProblem("Attribute definition '"+mName+"': "+msg);
          return null;
      }
docs: Method called during parsing of DTD schema, to report a problem.
  Note that unlike during actual validation, we have no option of
  just gracefully listing problems and ignoring them; an exception
  is always thrown.