[xslt] xsltproc should (optionaly) error out on parser errors

[Stefan Kost <ensonic hora-obscura de>]

Hi,

a long time agao I had filed this bug:
https://bugzilla.gnome.org/show_bug.cgi?id=624167

it's a dependency on a gtk-doc bug for me. I can try making a patch. Does anyone
agree that it is a good idea. Can I change the behaviour, or should it be
optional (--fail-on-error).

Stefan