Remove QJS exception API

Remove QJS exception API

This API has been deprecated for a while. It's legacy stuff from
QtScript.

Until someone proves that QJSValue::isError() isn't sufficient to
handle JavaScript exceptions, we won't provide any additional API
for that.

Also removed QJSValuePrivate::lessThan(), which was using the
exception mechanism, but the function itself wasn't used anymore
(another remnant from the QtScript days).

Change-Id: I3dffc6a7835874153f90d25ae2a72c93ea6db39a
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@nokia.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>