Adding an empty plot when hvPlotExplorer errors

[hoxbro]

When working with the hvPlotExplorer, it keeps showing the last working plot. I think it is wrong to show the last working plot. The current configuration of settings did not output that plot, which could confuse users.

An example of an empty plot was added in #811 https://github.com/holoviz/hvplot/pull/811/commits/3e011b52886b3427f76164cefc80f56a4a5dc1a3 and then removed again to discuss it in this issue.

An example of an empty plot:

[MarcSkovMadsen]

You might make it event clearer if the plot shows an image of something that signals there is an error. Not too overwhelming as the user should see the error message above. But still.

You could also replace the plot with the error message.