extensionSystem: Don't log error name

logExtensionError() currently saves the error message by calling
toString() on the passed error. That's convenient as it allows to
pass a string instead of a "proper" error, but the result isn't
great for the common Error case: Its toString() method prefixes
the message with the error name, which usually is just "Error:".

The plain message is more suitable for displaying it to users,
so use that for Error objects.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2337

js/ui/extensionSystem.js

@@ -260,7 +260,8 @@ var ExtensionManager = class {
         if (!extension)
             return;
 
-        let message = error.toString();
+        const message = error instanceof Error
+            ? error.message : error.toString();
 
         extension.error = message;
         extension.state = ExtensionState.ERROR;