[Feature Request] Allow component developers to expose custom errors / warnings in Dash Devtools

[Marc-Andre-Rivet]

As encountered in plotly/dash-core-components#729, it would be useful for component developers to expose degradation warnings / errors, etc. in Dash Devtools.

As currently implemented, only front-end errors thrown from within the React Error Boundary components are caught / handled by the Dash Devtools, making it impossible to both expose warning / error information to the user and rendering a component at the same time.

[wbrgss]

I think I ran into that problem myself. If it helps (but basically echoing this issue description):

I would prefer to throw a warning and still render the [component], but I can't figure out a way to display a Dev Tools warning without throwing an exception, and any uncaught exceptions will result in the component failing to render. Any caught exceptions (even with React Error Boundaries & componentDidCatch()) won't show the warning in Dev Tools. As an escape hatch I added a suppress_layout_exceptions flag to [the parent component]

[T4rk1n]

This has been implemented with set_props({_dash_error: new Error("custom")})