If a camera is not started successfully error() signal has to be
emitted.

This change adds a test for checking that we emit
and set the correct status and state when trying
to start an already active camera.

Task-number: QTBUG-51825
Change-Id: Ia14119aa9a93a76e363214401b8c2e9f216e35c9
Reviewed-by: Christian Stromme <christian.stromme@qt.io>