Choice fields should catch LookupError, not just VocabularyRegistryError, when getting a named vocab

[jamadden]

LookupError is the documented exception that IVocabularyRegistry.get is supposed to raise. VocabularyRegistryError is a subclass of that used by the specific default implementation. This means that when zope.vocabularyregistry is used, the generic LookupError from zope.component propagates.

Moreover, when Choice does catch VocabularyRegistryError it just raises a plain ValueError, with no indication of the failing field, value, or name. It should raise a ValidationError with the field and value. For backwards compatibility, this should also subclass ValueError and LookupError.