Avoid panicking when the resolver thread encounters a closed channel (#6182)
Closes https://github.com/astral-sh/uv/issues/6167
We've been seeing intermittent failures in CI, which we thought were
unexpected HTTP 401s but it actually looks like a panic when handling an
expected HTTP error. I believe the problem is that an early client error
can cause the channel to close and we crash when we unwrap the `send`.