Doctree validation: custom exception `nodes.ValidationError`.
The custom `ValidationError` has the additional attribute "problematic
element" that holds the element close to the violation ("self", for
invalid XML attributes and spurious text or a child node that does not
fit in the content model).
Use for warnings with source-code line
in `transforms.universal.Validate`
Revise validation error messages.
git-svn-id: http://svn.code.sf.net/p/docutils/code/trunk@9723 929543f6-e4f2-0310-98a6-ba3bd3dd1d04