How should we handle errors (e.g. expressions that don't succeed)?

[agoose77]

• Should we clear the cell attachments before requesting new values from the kernel?
• Should we delete failed keys from the attachments after updating from the kernel?
• Should there be a visual indicator, and if so, should it be different to the existing "no attachment" icon?

[fcollonval]

* Should we clear the cell attachments before requesting new values from the kernel?

It makes sense to ensure the values displayed are consistent with the same call to the kernel.

* Should we delete failed keys from the attachments _after_ updating from the kernel?

I think we should keep some value for error for other tools (like nbconvert,...).

* Should there be a visual indicator, and if so, should it be different to the existing "no attachment" icon?

It would be great to be able to get the error from the kernel if possible to help correcting the expression.

[agoose77]

#23 takes care of storing the traceback information where expressions fail, and we now clear the (jupyterlab-imarkdown) attachments before asking the kernel for new values.

At present, the errors are just displayed in-line.