Original errors are discarded when using web-sys FetchService

[jstarry]

Problem
The web-sys implementation of FetchService overwrites errors from building and sending requests which makes it very difficult to debug and figure out what is actually causing the error.

Expected behavior
Errors are not overwritten, and are bubbled up in a nice way using thiserror

[hamza1311]

FetchService is no more so I think we can close this.