ServerSetupObserver leaks changes if exception is thrown

If class used as @ServerSetup throws exception from callback, ServerSetupObserver purges all tasks that already have been executed. This means that all changes that have been introduced to AS model will remain( iirc persisted as part of config used my arquillian).
Hence other tests may fail due to that.

Ive talked with Stuart and according to his statement @ServerSetup class must not throw exception from callbacks.

ServerSetupObserver should cleanup in a more graceful manner.