Presentation of DTD validator output

[Klortho]

Reformat DTD validation error output, with links into listing. Right now, xmllint reports DTD validation errors like this:

not-valid.xml:9: element foo: validity error : No declaration for element foo

            ^
not-valid.xml:10: element journal-meta: validity error : Element journal-meta content does not follow the DTD, expecting (journal-id+ , journal-title-group* , (contrib-group | aff | aff-alternatives)* , issn+ , issn-l? , isbn* , publisher? , notes* , self-uri*), got (foo )

                   ^

The line number is in the error message, and the column number is implicit in the line with the carat. If it is consistent in this output, we can use it to identify the locations in the input file, in the same way that the Schematron validation output currently works.