Update get procedure for when UA does not support a protocol

[timcappalli]

After #401, we need to update the get procedure / processing rules for when a user agent does not support a protocol which is supported by the spec.

[MasterKale]

What about appropriating #249 for this?

[timcappalli]

What about appropriating #249 for this?

@marcoscaceres prefers smaller PRs

[marcoscaceres]

I think this is covered here in #420:

https://github.com/w3c-fedid/digital-credentials/pull/420/changes#diff-0eb547304658805aad788d320f10bf1f292797b5e6d745a3bf617584da017051R680-R682

If the user agent doesn't support the protocol, continue iterating. If after running through all of them, none are supported, {{TypeError}}.