When to fallback redirect?

In the current loading spec, redirect to the fallback URL happens only when the signed exchange version is not supported.

However, Chromium implementation performs fallback redirect in the following cases (corresponding to the error codes here):

• SXG was served from non-secure origin.
• Unsupported version of SXG (could extract fallback URL).
• SXG parse error (could extract fallback URL).
• Network error occurred while loading SXG header.
• Failed to fetch certificate chain.
• Failed to parse certificate chain.
• Signature verification failed.
• Cert verification failed.
• CT verification failed.
• OCSP check failed.
• Certificate Requirements aren't met.
• SXG was served without X-Content-Type-Options: nosniff header.

Chromium doesn't fallbeck on the following errors:

• SXG parse error (couldn't extract fallback URL).
• Merkle integrity error.

We should clarify that which error should cause a fallback redirect, and spec them.

Start with the loading spec's mp-http-fetch section and Chromium's signed_exchange_error.h, then compare the listed fallback and non-fallback cases. Done means the specification explicitly defines which errors trigger fallback redirects, including cases where the fallback URL cannot be extracted.