Fix removing tunnels when the tunnel factory throws an error

[joshaber]

If the tunnel factory in the web throws an error, VS Code will catch that error but maintain a port mapping for that port, which means if you try to forward the same port again, it won't re-call the tunnel factory since it thinks it's already forwarding the port.

Steps to reproduce

1. The tunnel gets added to the tunnel service's map:

   vscode/src/vs/platform/tunnel/common/tunnel.ts

   Line 468 in e7ee47c

   this.addTunnelToMap(remoteHost, remotePort, tunnel);

2. It will catch the error and return undefined: https://github.com/microsoft/vscode/blob/e7ee47c5ec2046533cb9017d7c403b32e2bfebd2/src/vs/workbench/contrib/remote/browser/tunnelFactory.ts#L65C1-L68
3. If the tunnel promise ends up resolving to an empty value, it will try to remove the empty mapping:

   vscode/src/vs/platform/tunnel/common/tunnel.ts

   Line 344 in e7ee47c

   this.removeEmptyTunnelFromMap(remoteHost!, remotePort);

4. But as part of that it again tries to resolve the promise, but instead of await'ing the tunnel promise it awaits the object from the tunnel map that contains the promise:

   vscode/src/vs/platform/tunnel/common/tunnel.ts

   Line 419 in e7ee47c

   const tunnelResult = await tunnel;

5. So it doesn't actually remove the port mapping since the result isn't undefined:

   vscode/src/vs/platform/tunnel/common/tunnel.ts

   Line 420 in e7ee47c

   if (!tunnelResult) {

cc @alexr00

[alexr00]

@joshaber, this is great. Thank you for debugging into this and for the details of your investigation!