When associating a DTD through <?xml-model...?>, DTD-related errors should be aggregated

[datho7561]

Please refer to issue #768 and this comment on the PR that addresses it.

When a DTD is associated with a file through <?xml-model...?>, and errors exist on the associated DTD, an error message should be shown. The current behaviour is that a "file not found" error is given:

This error should instead indicate that the DTD is invalid. Ideally, this error should also be an aggregation of the DTD errors like in #768.

Quick example to reproduce this issue:

ReferencesBorked.xml:

<?xml version="1.0" encoding="UTF-8"?>
<?xml-model href="Borked.dtd" type="application/xml-dtd"?>
<root>
</root>

Borked.dtd:

<!ELEMENT root (branch+)>
<!ELEMENTbranch (#PCDATA)>

[angelozerr]

Fixed with #916