Health paper: Intentionally failing cells

[vidartf]

I read / skimmed the health paper, and found it quite interesting. Having worked on nbval, I just wanted to share this point:

There is now a cell tag for hinting an expected failure of a cell, that is supported by both the notebook frontend and nbval: raises_exception. You could use this to either:

• Not count an exception in a cell as a failure.
• Count a lack of an exception as a failure.

There is some ambiguity about which way to interpret it. nbval uses its 'lax' flag to distinguish the two cases.

[somedave]

Thanks for the great info @vidartf - we'll take a closer look at this.