gateway: unify root identifier resolution errors

[lidel]

Extracted from ipfs/kubo#9660 (review) so we dont forget

I was thinking about this again, both from semantic perspective, but also given your feedback on how hairy this gets.
I am leaning towards returning 500s for all invalid root identifiers, just for the sake of consistency and simplicity

@hacdias would like to hear your thoughts if below makes sense (if you dont have any concerns, feel free to execute on this):

• /ipfs/this-is-not-a-cid → we were thinking about 400 Bad Request, but it may be better/easier to return 500 Server Error, because maybe today we know this CID is not valid, but in 10 years the same software could be wrong? Maybe it is a valid CIDv8 that this old software can't recognize? 400 could be wrong, 500 will always kinda fit.
• /ipns/this-domain-does-not-exist → 500 (we dont know if domain does not exist, or was resolution error)
• /ipns/{cid-libp2p-key-that-cant-be-resolved} → 500 (if it is a valid key, resolution error, if not, we will fallbck to dnslink, and that also cant be proven negative)

TLDR

Return HTTP 500 Server Error for all /ip[nf]s/id failures.

TODO

• PR here: fix(gateway): return 500 for all /ip[nf]s/id failures #187
  • Side-quest: IsErrNotFound func probably should not have routing.ErrNotFound, because these errors are for /ipns/id, which is we always want to be 500.
• PR in Kubo and tests passing: feat(gateway): error handling improvements (500, 502, 504) kubo#9660
• PR against ipfs/specs, documenting this behavior Return 500 on resolution errors specs#380

[hacdias]

@lidel I completely agree. It's very hard to detect where these errors are coming from without changing a lot of libraries and ensuring we clearly bubble up the errors. I will work on the code to make all requests return HTTP 500 here.