Update http errors we're trying to catch for httr2

[jennybc]

Fragment of a slack conversation with @hadley:

... since the code was written, gh has switched from httr to httr2 so there are a few http_error_404 that should now be httr2_http_404

(I was reminded of this when looking at #1893 and #1894.)

[jennybc]

I just did a GET for a nonexistent repo and here are the classes of the error:

 - attr(*, "class")= chr [1:5] "github_error" "http_error_404" "rlang_error" "error" ...

which suggests that perhaps gh is still emitting errors with the classes we are currently expecting.

My gh version is 1.4.1, which is after the switch to httr2.

[hadley]

Ok we must've preserved the error classes in gh. I think we can close this then.