Timeout errors do not align with documentation

Our API docs over at https://js-compute-sdk.fastly.dev/docs/fastly:backend/Backend/ specify: * `connectTimeout` and `firstByteTimeout` will result in a `503` response after request abortion. * `betweenBytesTimeout` will result in the response received so far being considered complete and the fetch ending. What actually happens: * `connectTimeout` results in the `fetch` promise itself failing with a `NetworkError` (code `19`) and a message of `Connection timeout`. * `firstByteTimeout` I couldn't get to time out at all, so I don't know what it would actually do. * [ ] `betweenBytesTimeout` I still need to test, but I don't trust that we're doing exactly what the docs say, right now.