ErrorWidget as fallback when widgets models or views fail - Following up

[trungleduc]

References

Initially started in #2960 by @maartenbreddels, this PR resolves the conflicts in #2960 and updates the behavior of the failback widget.

Code changes

• Based on the loadModelClass and loadViewClass logic on old PR, this PR adds some more try-catch blocks to catch the errors in model creation or view rendering.
• Add tests for the new error widget.

User-facing changes

• An error widget is shown on the failed widget, users can click on it to show the error messages.
• The layout of the parent widget is preserved in case of failed children widgets.

138243743-34a49956-6d54-4aab-ba7d-12713a12c762.mp4

Backwards-incompatible changes

N/A

[github-actions]

👈 Launch a binder notebook on branch trungleduc/ipywidgets/feat_error_widget

[trungleduc]

Build extension failed because the Jupyterlab browser_check issue (jupyterlab/jupyterlab#11345)

[martinRenou]

Note: I made a half-baked backport of this PR to the 7.x branch for private needs, putting the link here in case we need it later. Meeseeks will not be able to backport it automatically.