Issue Type: Bug

1. Use an extension that implements a FileSystemProvider for which you can alter conditions so that it may or may not resolve. For example, an FTP provider connecting over a VPN that you can disable.
2. Have a workspace that uses the FSP.
3. Be able to resolve the workspace folder and see its contents.
4. Change environment state to make the target unreachable (e.g. disconnect VPN)
5. Use the 'Refresh Explorer' button. The Explorer correctly displays the root folder with a '!' indicator and colour, plus a hover tip saying 'Unable to resolve workspace folder'
6. Change environment state to make the target reachable again (e.g. connect VPN)
7. Use the 'Refresh Explorer' button.

🐛 the root folder retains its error indicator, colour and tip even though the folder now resolves and expands.

I will submit a PR

VS Code version: Code - Insiders 1.58.0-insider (6afedfd, 2021-06-21T05:15:13.618Z)
OS version: Windows_NT x64 10.0.19043
Restricted Mode: No