Allow yellow "unstable" state from a step

The xUnit plugin allows for a yellow "unstable" state, between red "FAIL" and blue "OK". This is useful for test-suite steps, to help developers immediately distinguish between "I broke one of 40,000 unit tests" and "I caused compilation to fail entirely".

It would help if there were an easy way to assign an unstable state as the result of a step, especially a shell step (e.g. if we could check for a magic exit code to mark a step as unstable rather than failed)