Timeout errors do not align with documentation

[zkat]

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 DOMException (name NetworkError, code 19, message Connection timeout).
• (TODO) firstByteTimeout I couldn't get to time out at all, so I don't know what it would actually do.
• (TODO) betweenBytesTimeout I still need to test, but I don't trust that we're doing exactly what the docs say, right now.