mod_http_oauth2: Fix returning errors from response handlers This would either redirect the user back to the client along with the error code, or show the error HTML template. Previously this would just show some JSON to the user.

This module blocks configured queries that change server state.

E.g. to make vCard storage read-only:

``` lua
readonly_stores = {
	vcard = { "vcard-temp", "vCard" };
}
```