refactor!: ErrorExt::raise_iter to raise_all + remove Frame::downcast

[tisonkun]

This should be included in #2384, but I failed to find it at that time :\

[tisonkun]

The doc comment should be "Try to downcast this error as a ref to the exact type T, or return None"

But since it can be written as a fluent call f.error().downcast_ref::<T>(), I tend not to add this method to keep the API less.

[Byron]

Thanks you, this looks great!

Happy to shed this extra method as well.

[Byron]

@tisonkun Please note that I think it's best for me to see the conversion through to the end, and by that time I'd expect gix-error::exn to be stable enough to look at migrating back to exn again.

[tisonkun]

@tisonkun Please note that I think it's best for me to see the conversion through to the end, and by that time I'd expect gix-error::exn to be stable enough to look at migrating back to exn again.

Ack. Try to fill the gap in #2351 (comment).