Fixes issues with 'firefox extension needed' error

JitsiTrackErrors.js

             devices.indexOf("video") !== -1) {
                 return this.UNSUPPORTED_RESOLUTION;
         } else if(typeof error === "object" && error.type === "jitsiError") {
-            return error.object;
+            return error.errorObject;
         } else {
             return this.GENERAL;
         }

modules/RTC/ScreenObtainer.js

 
         // Make sure desktopsharing knows that we failed, so that it doesn't get
         // stuck in 'switching' mode.
-        errorCallback(JitsiTrackErrors.FIREFOX_EXTENSION_NEEDED);
+        errorCallback({
+            type: "jitsiError",
+            errorObject: JitsiTrackErrors.FIREFOX_EXTENSION_NEEDED
+        });
     },
     /**
      * Asks Chrome extension to call chooseDesktopMedia and gets chrome