Handle non-200 status codes when downloading WEBC files

[Michael-F-Bryan]

I've refactored the wasmer_wasix::http module to use more types from the http crate. This lets us implement a HttpResponse::is_ok() method in terms of the http::StatusCode we get back, rather than making each client implementation decide whether a request failed or not.

That means when a package has recently been released and the backend hasn't finished generating a *.webc file for it (which we get a 503 for), instead of seeing an error like this:

$ wasmer run-unstable https://wapm.io/wasmer/wasmer-io
error: Unable to resolve "https://wapm.io/wasmer/wasmer-io"
│   1: Unable to detect the WEBC version
╰─▶ 2: Expected the magic bytes to be "\x00webc", but found "The w"

You will see something like this:

$ wasmer run-unstable https://wapm.io/wasmer/wasmer-io
error: Unable to resolve "https://wapm.io/wasmer/wasmer-io"
│   1: GET request to "https://wapm.io/wasmer/wasmer-io" failed
|   2: 503 Service Unavailable (Retry after: 5 minutes)

[theduke]

Nice!