Assertion checking in workflows

[tnabtaf]

I would like to have the ability to do assertion checking in workflows, and if the assertion fails, stop the workflow. A couple of assert examples that would have helped me:

1. I want to check that the values in a particular column are unique. If they aren't stop the workflow.
2. Check that a dataset is not empty. If it isn't, stop the workflow.

One of those tests looks at values in the dataset. Another looks at the dataset as a whole. Not sure how best to implement assertions, how complex they can get, and what sorts of things can they test.

It would be nice to have the assertion display user-provided text if an error occurs.