extensions: Consistently handle createExtensionObject() errors

The method may throw an error, for example when metadata.json is
missing or cannot be parsed, however we are currently not always
handling it.

https://bugzilla.gnome.org/show_bug.cgi?id=781728

js/ui/extensionSystem.js

@@ -199,7 +199,14 @@ function reloadExtension(oldExtension) {
     unloadExtension(oldExtension);
 
     // Now, recreate the extension and load it.
-    let newExtension = ExtensionUtils.createExtensionObject(uuid, dir, type);
+    let newExtension;
+    try {
+        newExtension = ExtensionUtils.createExtensionObject(uuid, dir, type);
+    } catch(e) {
+        logExtensionError(uuid, e);
+        return;
+    }
+
     loadExtension(newExtension);
 }