Currently, the first time Coder Desktop dials /api/v2/tailnet is within the main Controller.Run loop:

This means the tunnel sends the StartResponse with success set to true without knowing whether or not it was able to dial the coordination endpoint. This results in a poor UX if we're not able to dial it initially; Coder Connect appears functional but is not - the only sign something went wrong is the No workspaces message, and N error logs, for each retry attempt.

Instead, we should fail fast, and send a StartResponse with success set to false if we fail to dial right after starting.