Ability to differentiate between fetch error and timeout

Right now, timed-out fetches (configured via timeoutMSecs) just run the onerror callback: https://github.com/emscripten-core/emscripten/blob/main/src/Fetch.js#L386. We'd like to be able to differentiate between failed fetches and timed-out fetches, preferably by exposing the ontimeout part of the underlying XHR.