HTTP error responses implementations inconsistent with spec #191

As discussed on the 2025-08-28 WebPerf WG call it appears there are differences between the error codes in the spec, Chrome, and Firefox at least.

The spec has the following:

• http.response.invalid
• http.response.redirect_loop

Chrome has the following:

• http.response.invalid
• http.response.invalid.empty - not in spec
• http.response.invalid.content_length_mismatch - not in spec
• And http.response.redirect_loop may not be used (or is in a different code path)

Firefox:

• http.response.redirect_loop
• Does not use http.response.invalid but instead returns unknown-ERROR-NUMBER (where ERROR-NUMBER is a number). Which is not spec compliant but maybe more useful (unless we add a lot more reasons to the spec so that all error codes can be mapped to one of these).

We should discuss on what is the best option here and update all three to match that. Let's discuss here, and maybe discuss at TPAC 2025?

FYI: @Bas-moz @pmeenan @rmarx @yoavweiss @nicjansma