HTTP/3 panic if udp is not available

[NobodyXu]

From cargo-bins/cargo-binstall#1292 :

192.1 thread 'main' panicked at 'unable to create QUIC endpoint: Os *** code: 92, kind: Uncategorized, message: "Protocol not available" ***', /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/reqwest-0.11.18/src/async_impl/h3_client/connect.rs:41:58

[NobodyXu]

The user run this in docker and errno 92 means protocol not available, meaning udp not available.

[NobodyXu]

Given that it's not unheard of for udp to be banned, and now it seems to be banned within docker, I think it's a good idea to make it "Option<>"nal.

[seanmonstar]

We should change it to return an error at least, instead of panic. An eventual feature improvement could allow falling back to other protocols, but only if someone configures they want to allow that; http3_prior_knowledge implies "it MUST use http3". Panics are still bad, though!

[NobodyXu]

An eventual feature improvement could allow falling back to other protocols, but only if someone configures they want to allow that

IMO fallback to other protocol should be the default, our user of cargo-binstall discovers this when running cargo-binstall inside docker and is probably blocked by the seccomp filer docker installed.

Also many corp env do the same and block udp, so fallback should be the default unless turned off by http3_prior_knowledge.