Add error for when the API is supported, but the platform doesn't support it

[marcoscaceres]

There are cases where the browser engine supports the API (e.g., it knows about options.digital) but the underlying platform doesn't implement the APIs needed to perform a presentment flow.

In such cases, the spec should say to reject with a NotSupportedError.

[timcappalli]

We just need to be careful with wording, because some browsers will implement the cross-device presentation/issuance protocol directly, which does not require the underlying platform to understand anything related to the Digital Credentials API.