Clarify non-terminal error codes for FETCH and ANNOUNCE

[englishm]

Discovered while writing #638

• FETCH_ERROR
• SUBSCRIBE_ERROR
• ANNOUNCE_ERROR

Each of these messages has a field for an error code, but we don't currently appear to have any specific error codes defined other than reasons for terminating the entire session: https://moq-wg.github.io/moq-transport/draft-ietf-moq-transport.html#name-termination

Edit to add: I found the table for SUBSCRIBE_ERROR:
https://moq-wg.github.io/moq-transport/draft-ietf-moq-transport.html#section-5.1-8

Also, we do have this text for Fetch:

If StartGroup/StartObject is greater than the latest published Object group, the publisher MUST return FETCH_ERROR with error code 'No Objects'.

but I don't see such an error code defined anywhere yet

[afrind]

I think this was addressed by #656