StaticExtension does not play well with dp inheritance for ErrorHandling.Mode.

[JohanLarsson]

Checked in test NoTranslationsWithReturnErrorInfo with repro.
My guess is that the static extension is called before the inheritance has trickled down when the control is buried deep in the visual tree.

A couple of alternative solutions:

• Defer a check with DispatcherPriority.Loaded
  • Implement a subscription mechanism that updates the binding when ErrorHandling.Mode changes.