Shorten on_failure key on run configuration to failure

Rename the on_failure key on the run configuration to just failure. The snake casing makes the key look different than all other keys (which are a single segment) and is longer to type. But the most important part to me is how it reads. If the key is failure: 'ignore', that's a postfix way of saying "ignore failure". We can keep on_failure as an alternative form.

As part of this change, drop the old onFailure that it used to be, which was even more inconsistent.