AJAX responses do not support exceptions messages

The updated http_exception and runtime_exception classes added in 3.1.3 do not fully support AJAX responses.

When an exception is thrown in during an AJAX process, the response displays to the user an error message based on the status code.

The custom message string, if supplied in the thrown http/runtime_exception should be displayed to the user in the AJAX error window.

This allows extensions, for example, using AJAX to provide meaningful error messages to users besides simply "Forbidden" or "Internal server error" just as we already can do for non-AJAX based exceptions.