Improve ClosedError handling in QGeoPositionInfoSource

The previously described behavior was introduced by a platform
which QtPositioning no longer supports. Effectively this is a
behavior change which currently doesn't affect any known backend.

The upcoming Android & BB10 backends support this new behavior.

The described behavior was very inconvenient from a QML developer's
perspective. Deleting a QML PositionSource is rather inconvenient and
difficult to do. The API should make a developer's life easier.

Task-number: QTBUG-33457
Change-Id: I94432c0569b647cd33c27f75ed73ce5322f40d47
Reviewed-by: Anders Gunnarsson <anders.gunnarsson@appello.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>