Document allowing errors with cell tags

[choldgraf]

Maybe I'm missing it, but I couldn't find the ability to mark cells as "raises errors" with tags:

https://github.com/jupyter/nbclient/blob/507876fa03a71d2461e273ba4d9ec12d3a9ea881/nbclient/client.py#L543

if this is a feature we want to support (I think it'd be useful) we should document it!

[akhmerov]

I find this a bit of a tricky question: this tag is not a part of the official spec, and nbclient should probably follow the spec. At the same time, the control over the errors seems somewhat useful compared to a binary per-notebook switch. The usefulness is limited though, since one can afterwards inspect the executed notebook and check that only tagged cells produced errors.

[MSeal]

So there were many long threads on this topic in the past:

https://github.com/jupyter/nbconvert/issues/680 https://github.com/jupyter/nbconvert/issues/730 https://github.com/jupyter/nbconvert/issues/838 https://github.com/jupyter/nbconvert/pull/684

Essentially, every interface supported this at the time it was implemented except nbconvert and it was agreed that it was unofficially part of the spec (this pre-dates my direct involvment in jupyter a bit) and that it should be readdressed whenever the spec moved to 5.0.

At this point it should probably be documented in nbformat as part of 4.4 spec.