Avoid double recording in iOSDevice

Summary: This should in theory never happen since the state of the recording is also managed within the UI. However, it's better to ensure that if `startRecording` is called twice, without calling `stopRecording` first it will error rather than leak a recording (the first process will still run but the object representing it is oprhaned)

Reviewed By: passy

Differential Revision: D33913298

fbshipit-source-id: f8fa6b0e4fdbdf4282633fa29a736d3e7dedf3bb

desktop/flipper-server-core/src/devices/ios/IOSDevice.tsx

@@ -81,6 +81,12 @@ export default class IOSDevice extends ServerDevice {
   }
 
   async startScreenCapture(destination: string) {
+    const recording = this.recording;
+    if (recording) {
+      throw new Error(
+        `There is already an active recording at ${recording.destination}`,
+      );
+    }
     const process = this.iOSBridge.recordVideo(this.serial, destination);
     this.recording = {process, destination};
   }