Don't hide the start exception when failing to start and stop

[magnars]

When failing to start the system, the start exception is more interesting than the stop exception. This change prioritizes start-ex over stop-ex in the exception cause chain.

The start-exception is still included in the ex-data, to avoid breaking any code expecting to find it there.

[aengelberg]

I agree this would be a great change. :+1: