[XLA] Raise internal error on mismatched backend config descriptors.
[XLA] Raise internal error on mismatched backend config descriptors.

This is more consistent with the original goal of this function. Originally, it crashed if the descriptors mismatched. It was later refactored to avoid the crash and inadvertently changed to simply fall through instead of returning an error. This fallthrough introduced a subtle bug: If the descriptors are mismatching but the stored `proto_` is not empty, the stored `proto_` will be overwritten.
