std.http's one-line client calls lose the status: a 404 and a 500 both read "http error" #3147

http.get, get_with_timeout, post, put and delete turn any response that is not a 2xx into the error "http error" (and anything below 200 into "unexpected status"). The status is lost: a 404, a 500 and a 503 read the same, so a caller cannot tell "not found" from "the server is down" without moving to std.http.client. std/http/module.ae, get and its siblings.

Fix: the error names the status and its reason phrase, "HTTP 404 Not Found" ("HTTP 503 Service Unavailable"), for every one-line wrapper; the reference says so; tests check a 404 and a 500 from a local server. std.http.client stays the API that hands back the whole response.

Found on #3101's successor batch (#3105, writing the HTTP example).