Informative error if mis-naming the top-level default class.

Also:
- Move the the top-level default class name into the mjCModel constructor.
- Fix bug in default class propagation in frame elements.
PiperOrigin-RevId: 646384669
Change-Id: Icc6154124361a234b84a07e2b5ecaa7c8b18ebf7

src/user/user_model.cc

@@ -97,6 +97,7 @@ mjCModel::mjCModel() {
 
   //------------------------ master default set
   defaults_.push_back(new mjCDef);
+  defaults_.back()->name = "main";
 
   // world body
   mjCBody* world = new mjCBody(this);