cam-app-example: Set failure exit code when stream fail

Currently the cam-app-example doesn't set a failure exit code
when one of the stream fails, fix that setting a proper failure
code.

Fixing this bug revealed that the test
'test_cam_app_invalid_event_logic' that is supposed to trigger an
error on the stream was using the wrong logic to test the app exit
code, update it to assert that the app fails.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>