check for tflite model before enabling cam

templates/browser-detect.html

@@ -761,8 +761,9 @@
         }
         // Enable the live webcam view and start detection.
         function enableCam(event) {
-            if (!handLandmarker) {
+            if (!handLandmarker || !letterDetector) {
                 console.log("Wait! objectDetector not loaded yet.");
+                document.getElementById('webcamButton').innerHTML='<span>Retry</span>'
                 return;
             }
             if (webcamRunning === true) {