MissingCategoryDocumentConstraint was incorrectly using
unrestrictedTraverse, unlike category API which uses its own
resolveCategory method with better semantics.

Because of this, some problems were not reported:
 - categories using paths that resolve by acquisition, but does not
exists as such (like in the test)
 - categories using path with dynamic objects, like a website language.

/reviewed-on nexedi/erp5!986