Enhance Jupyter Preview Error Message if notebook raise exception

[MarcSkovMadsen]

A colleague needed my help to solve an issue with the Jupyter Panel Preview.

The issue could be minimized to

I.e. panel is not imported and the notebook cannot run end to end.

But the error message in the Jupyter Panel Preview does not report this exception. Instead it shows some text that could lead me or my colleague in the wrong direction.

Please show the exception.

Reproducible Example

pn.extension()

pn.panel("Hello World").servable()

[philippjfr]

Worth noting that as of right now, i.e. on main, it will fall back to showing the dashboard builder UI. Before releasing 1.4.0 we should instead detect exceptions and display those as suggested.