Issue Type: Bug

1. Clone vscode-extension-samples
2. Open fsprovider-sample folder
3. Add the following lines to the top of writeFile in src/fileSystemProvider.ts

        if (uri.path.startsWith("/.")) {
            throw vscode.FileSystemError.NoPermissions("dot-folders not supported by MemFS");
          }

4. Run the sample.
5. In the EH window, run the MemFS: Setup Workspace command.
6. Open Settings, select the subtab for the 'MemFS - Sample' folder, then use the action button to open JSON.

Result is the following notification:

I assume the text of the thrown error should appear instead of {1}

VS Code version: Code 1.50.0 (93c2f0f, 2020-10-07T06:10:52.432Z)
OS version: Windows_NT x64 10.0.18363