Return error instead of panicing when server not ready

I'm running into an `assert!(...)` here, which should probably just return an error, instead: https://docs.rs/async-h1/2.1.2/src/async_h1/client/decode.rs.html#32

(I think that's what the `TODO` there is suggesting.)

In this case, I know that the server I'm querying may not be up yet -- I'm issuing requests in a loop with a timeout, for the purpose of waiting for the server to come up all the way.

Start in async_h1/client/decode.rs around the assert at line 32 and read the nearby TODO. Reproduce a request while the server is unavailable, then verify that this path returns an error rather than panicking. Done means the reported not-ready-server case no longer triggers the assertion.

Written by the indexing model from the issue text.