safesocket: reduce log spam while running integration tests

Instead of logging lsof execution failures to stdout,
incorporate them into the returned error.

While we're here, make it clear that the file
success case always returns a nil error.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

safesocket/unixsocket.go

@@ -28,16 +28,15 @@ func connect(path string, port uint16) (net.Conn, error) {
 	pipe, err := net.Dial("unix", path)
 	if err != nil {
 		if runtime.GOOS == "darwin" {
-			extConn, err := connectMacOSAppSandbox()
+			extConn, extErr := connectMacOSAppSandbox()
-			if err != nil {
+			if extErr != nil {
-				log.Printf("safesocket: failed to connect to Tailscale IPNExtension: %v", err)
+				return nil, fmt.Errorf("safesocket: failed to connect to %v: %v; failed to connect to Tailscale IPNExtension: %v", path, err, extErr)
-			} else {
-				return extConn, nil
 			}
+			return extConn, nil
 		}
 		return nil, err
 	}
-	return pipe, err
+	return pipe, nil
 }
 
 // TODO(apenwarr): handle magic cookie auth