cmd/tailscale: have web POST wait for authURL

Fixes #1939

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>
(cherry picked from commit 82b217f82e)

cmd/tailscale/cli/web.go

@@ -321,6 +321,10 @@ func tailscaleUpForceReauth(ctx context.Context) (authURL string, retErr error)
 	})
 	bc.StartLoginInteractive()
 
+	<-pumpCtx.Done() // wait for authURL or complete failure
+	if authURL == "" && retErr == nil {
+		retErr = pumpCtx.Err()
+	}
 	if authURL == "" && retErr == nil {
 		return "", fmt.Errorf("login failed with no backend error message")
 	}