Promote warnings to errors in extract()

[Girgias]

Split from #4566

[theodorejb]

Is the return type still int|null? If it is no longer nullable, the proto comment should be updated as well as the stub signature.

[Girgias]

Is the return type still int|null? If it is no longer nullable, the proto comment should be updated as well as the stub signature.

I don't think it can return null any more so I've updated the stubs.

[nikic]

Actually, please drop the extra variation tests. The only thing we need is to check the invalid prefix case in the existing error test, the rest just duplicates existing tests.

[Girgias]

Yeah I forgot those were covered in the normal error ones.

Removed them and merged as 70e604e