Supress error dialog if update checker fails with no message

If there's no internet connection, for example, the dialog would be
displayed with an empty error message, which isn't helpful.

src/gui/html/js/updateExists.js

@@ -55,6 +55,12 @@
 
         return !tag.commit.sha.startsWith(currentBuild);
       });
+    }).catch((error) => {
+      if (!error.message) {
+        console.error(error);
+      } else {
+        throw error;
+      }
     });
   };
 }));