Allow client widget drivers to serialize Matrix API error responses into
JSON to be received by the requesting widget.

because Typescript has its own version of that rule

Mock client-side responses to test deserializing them on the widget side

As long as the error details payload is extensible, let drivers put more
data in them than just the key for Matrix API error responses.

as this makes it too easy for drivers to put data in the wrong section.

Still define the payload type as an interface so that it can be
extended in a future version of the API.

Also don't use a subfield now that non-extensibility makes the format of
the details fields unambiguous.