JSONP error handling

When a request has set the JSONP query parameter we always respond with an HTTP 200 OK.
If an error occurred (e.g. not all required query parameters were set) the error message is set as 'data' in the JSONP callback.
The error message is the same as the error message in an ordinary request. Additionally, the error message contains an attribute 'http_status'.

(Created a dedicated 'settings' module and moved some of the default settings to the new module.)