WIP: Improve error reporting in ext_client_setup. Fail, if there is a mismatch when using struct_info.

This is my first iteration on the topic. It works (good), but is not overly pretty. Not pretty, because it respects the constness of the struct_info inside the match function. And then in case of failure sets the things anyways. This will fail hard on the first bad match. Another idea is to have a flag to tell it to not care about bad matches and run through as before. Then, Håkan's solution proposed via mail is of course more elaborate.