Surface errors in notebook cells not visible on the page

[stevejpurves]

Context

When articles can display notebook outputs as figures, or if notebooks support hidden cells - it is possible that an error will occur in a notebook cell that is not attached to the page, so the error output will not be visible.

Proposal

Implement improvements to the error handling to catch errors that occur in the notebook and surfaces them to the UI. The error handling should be consistent with the extended multi-page execution scope introduced in #136 and probably should be implemented via a new provider (similar to the busy provider) in conjunction with the existing ErrorTray component.

Tasks and updates

No response