Custom editor doesn't handle extension errors well

re #93767

I have declared `readonly uri: vscode.Uri` in my `FooCustomDocument` which means an errors happens while resolving. The editor just appears as dead, no message for the user

<img width="1260" alt="Screenshot 2020-03-31 at 18 48 33" src="https://user-images.githubusercontent.com/1794099/78053183-830e9300-7380-11ea-91c1-870b259cab68.png">
