Debugging a subnotebook

[zmbc]

I'm not sure the best way to design this, but want to raise an idea for a potential feature: the ability to "debug" a subnotebook the same way you can "debug" a Jupyter notebook.

Here's one concrete way I can imagine this working, which would be clunky, but illustrates the aim: If an exception is encountered in a subnotebook, an actual notebook file is saved containing the subnotebook and its execution progress so far, and the running kernel (after the exception) is somehow automatically associated with the notebook, so that the user can open that new notebook in Jupyter and interactively run cells right after the failure.

Then, there could be a subnotebook equivalent of pdb.set_trace() that triggered this behavior without an exception.

Thoughts @davidbrochart? Do you think this is a reasonable idea, or misguided in some way?