Standards compliance concerns raised by Dominic in Spec #829

This issue tracks standards compliance concerns raised by domfarolino in the discussion of PR #815 (Enabling IDP Interception in FedCM Request):

Summary of Concerns:

1. "[=Fetch request=] with |request| and |globalObject|, and with
   processResponseConsumeBody set to the following steps given a
   response |response| and |responseBody|:"
   comment by domfarolino
   Let's specify the type of |responseBody| here in the declaration of these steps. Per https://fetch.spec.whatwg.org/#ref-for-process-response-end-of-body%E2%91%A0 it should be a null, failure, or byte sequence.

2. " [=create an IdentityCredentialError|creating an IdentityCredentialError=] with {} "
   comment by domfarolino
   [=create an IdentityCredentialError|creating an IdentityCredentialError=] with «»
   See how the Infra Standard defines list literal syntax. I see there are other pre-existing instances of this in this spec already, but let's start incrementally using the right one.

3. " The user agent MAY set |credential|'s {{IdentityCredentialError/error}} based on
   |response|'s [=response/status=]. For example, if the [=response/status=] is 500, it
   could set it to "server_error", and if the [=response/status=] is 503, it could set"
   comment by domfarolino
   It seems a little unfortunate that this is not standardized. Has there been any discussion about doing that?

Refer to PR #815 for full context.