Use special JitsiTrackError object instead just strings for various types of errors that may happen to JitsiTrack

conference.js

                 this.videoSwitchInProgress = false;
                 this.toggleScreenSharing(false);
 
-                if(err === TrackErrors.CHROME_EXTENSION_USER_CANCELED)
+                if (err.name === TrackErrors.CHROME_EXTENSION_USER_CANCELED) {
                     return;
+                }
 
                 console.error('failed to share local desktop', err);
 
-                if (err === TrackErrors.FIREFOX_EXTENSION_NEEDED) {
+                if (err.name === TrackErrors.FIREFOX_EXTENSION_NEEDED) {
                     APP.UI.showExtensionRequiredDialog(
                         config.desktopSharingFirefoxExtensionURL
                     );
                     return;
                 }
 
+                // TODO: handle Permission error
+
                 // Handling:
                 // TrackErrors.CHROME_EXTENSION_INSTALLATION_ERROR
                 // TrackErrors.GENERAL

nwjs-integration/index.html

                         };
                         navigator.webkitGetUserMedia({
                             audio: false, video: vid_constraint
-                        }, callback, errorCallback);
+                        }, callback, function (error) {
+                            errorCallback &&
+                                errorCallback(error, vid_constraint);
+                        });
                     }
                 );
             }