[suggestion] catch invalid XML in listings

[line-o]

Is your feature request related to a problem? Please describe.

As #549 shows it is possible to introduce listing data containing invalid XML.
This will then lead to this listing not being stored on installation in existdb leading to inconsistent documentation.

Describe the solution you'd like

I would like to have a check in place that will warn the editor that a listing is not valid XML.

Describe alternatives you've considered

Alternatively it could be done in a pre-commit hook that will not allow adding invalid XML to version control.

Additional context

This is what a user sees when installing a documentation with invalid XML.

[duncdrum]

As per the Author guidelines, listing.xml should always be well-formed, otherwise xml should be stored as listing.txt. Not well-formed xml fragments as listings can improve article quality by removing clutter, and getting straight to the point.