administration: fix MissingCategoryDocumentConstraint with acquisition

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.