mod_http_oauth2: Show errors on device flow user code entry page If the user enters the code incorrectly, having to click back to try again is no fun. Instead, show the error and the code entry form again.

This module blocks configured queries that change server state.

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

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