cmd/tailscale: fail if tailscaled closes the IPN connection

I was going to write a test for this using the tstest/integration test
stuff, but the testcontrol implementation isn't quite there yet (it
always registers nodes and doesn't provide AuthURLs). So, manually
tested for now.

Fixes #1843

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/up.go

@@ -344,7 +344,8 @@ func runUp(ctx context.Context, args []string) error {
 
 	startingOrRunning := make(chan bool, 1) // gets value once starting or running
 	gotEngineUpdate := make(chan bool, 1)   // gets value upon an engine update
-	go pump(pumpCtx, bc, c)
+	pumpErr := make(chan error, 1)
+	go func() { pumpErr <- pump(pumpCtx, bc, c) }()
 
 	printed := !simpleUp
 	var loginOnce sync.Once
@@ -404,6 +405,8 @@ func runUp(ctx context.Context, args []string) error {
 	case <-gotEngineUpdate:
 	case <-pumpCtx.Done():
 		return pumpCtx.Err()
+	case err := <-pumpErr:
+		return err
 	}
 
 	// Special case: bare "tailscale up" means to just start
@@ -456,6 +459,8 @@ func runUp(ctx context.Context, args []string) error {
 		default:
 		}
 		return pumpCtx.Err()
+	case err := <-pumpErr:
+		return err
 	}
 }