The post-handler redirect logic was unconditionally converting responses
to 302 Found when the request had Accept: text/html and a Referer header.
This overwrote error status codes (4xx, 5xx), making it impossible to
return errors from #[get] endpoints accessed via browser navigation.
Add a response.status().is_success() guard so only successful responses
get the Post/Redirect/Get treatment.
Fixes#5428
John Costa
45442af41d