Some minor optimizations

Change-Id: Ib2e08e7c89ca59a48f8fd52b30981e5d7e60803b
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Remove internal method from public API

QQmlError is public API and shouldn't expose an internal
method.

Change-Id: I7caf06af9340fefec5c96103395fe74acbf19497
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>