Update remaining `errors.As` call sites to use `errors.AsType` (#9106)

Generic implementation > throwaway pointers.

With that in place, configure the `forbidigo` linter to flag all use of
`errors.As` from here on. If there's any reason not to that I missed, we
can of course revert it later. But at least no occurence in our current
codebase that wasn't easily replaced by `errors.AsType`.

Also some general error handling improvements while at it.

Signed-off-by: Anders Eknert <anders.eknert@apple.com>