Track unable to listen cases

Summary: Track whenever the server is unable to bind to the specified port.

Reviewed By: antonk52

Differential Revision: D49951304

fbshipit-source-id: 971d7c324f27c287036707d21bb51d26acbd6615

desktop/flipper-server-core/src/server/startServer.tsx

@@ -174,6 +174,7 @@ async function startHTTPServer(
       console.warn(
         `[flipper-server] Unable to listen at port: ${config.port}, is already in use`,
       );
+      tracker.track('server-socket-already-in-use', {});
       process.exit(1);
     }
   });