When deserializeWebviewPanel fails, we should be able to display an error message in a better way

[tamuratak]

When we can not restore Webview with WebviewPanelSerializer.deserializeWebviewPanel, we should be able to display an error message on each Webview, or in other much better ways.

Typically, when files we wanted to preview on Webview have been removed, we cannot restore the Webview. In those cases, the current API gives extension authors no way to tell users what went wrong. Although the authors can use showErrorMessage or showInformationMessage, it does not seem an appropriate way when multiple restoring fails at the same time. We should be able to tell users what went wrong on each Webview.

Since restoring Notebook, #95361, would easily fail, extension authors of Notebook would need this feature.

[mjbvz]

Can you explain what problem you are running into and the use case for this proposal?

Webviews are already free to show their own error message if they cannot restore themselves properly (such as if the file they were previewing no longer exists). However VS Code will show a generic error message if the webview throws an uncaught exception while restoring a webview

[tamuratak]

I missed that we could use Webview itself to display error messages.

WebviewPanelSerializer.deserializeWebviewPanel really works well. Thank you.