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.

[marcoscaceres]

Addressed by #395

[mohamedamir]

+1 to what @timcappalli said.
The platform doesn't necessarily support the API.
And as I commented on #395, why not handle that in the userAgentAllowsProtocol (), because it's the really the UA that decided not to allow this API.

I find it a bit misleading for a UA to define the DigitalCredentials interface but doesn't support the DC API.

[timcappalli]

I find it a bit misleading for a UA to define the DigitalCredentials interface but doesn't support the DC API.

I agree. If a minimum set of pre-reqs are not met, the UA should not enable the DigitalCredentials interface.