(bug 49264) Make SnakValidator fail on bad values.

Snaks that contain a UnDeserializableValue, or whose
value type doesn't match the respective property's DataType,
should be considered invalid.

IMPORTANT: requires Id27f19020 in WikibaseDataModel

Change-Id: I1e2543c75f0407a22e531aac67ace9cf3f5670ae
Merge "(bug 49264) Make SnakValidator fail on bad values."