Add option to report build as failed when tests do not pass

[ludofischer]

Some people prefer to mark the build as failed when the tests don't pass, as can be achieved with some other plugins.

[ikeike443]

Thanks.
But please test whether it will go fine, that means, it will keep the default behavior when some users who is using the current version let your option be blank.

[ludofischer]

The default behaviour (report Unstable) is selected unless the user checks the box.

Unfortunately, at least since 1.2.4 RC1 (maybe also 1.2.3), there is a separate issue with Play: play auto-test returns with status code 1 instead of 0 whenever tests fail, which causes the build to be marked as Failed instead of Unstable.