NFC: simplify internals of http::response::try_from_incoming

[pchickey]

This change eliminates some error-handling control flow in try_from_incoming which is unreachable, and replaces it with expect:

The http::response::Builder keeps internal state of whether the builder has errored, which is only reachable by passing Builder::header an erroring TryInto<HeaderName> or TryInto<HeaderValue>. Since the Builder::header method is never used, we know Builder::headers_mut will never give the None case, nor will Builder::body give the error case. So, rather than treat those as control flow, we unwrap if this invariant is ever broken because that would only be possible due to some unrecoverable bug in wstd, rather than incorrect use or invalid input.

This is a nonfunctional change and really just serves the next person who has to examine try_from_incoming and figure out exactly what error case it encountered, and save them reasoning through that those bits of control flow are actually unreachable.