Check that there are no pending keyframes while writing a spec.

This can happen when a body is detached without compiling or recompiling.

Also added error catching when trying to delete a body instead of detaching it.

Fixes #2327

PiperOrigin-RevId: 712526730
Change-Id: I4b48df83120fca12d475c85b3a54893841449653

src/user/user_api.cc

@@ -254,10 +254,17 @@ int mjs_activatePlugin(mjSpec* s, const char* name) {
 
 
 
-// delete object, it will call the appropriate destructor since ~mjCBase is virtual
-void mjs_delete(mjsElement* element) {
+// delete object, return 0 if success
+int mjs_delete(mjsElement* element) {
   mjCBase* object = static_cast<mjCBase*>(element);
-  object->model->DeleteElement(element);
+  try {
+    // it will call the appropriate destructor since ~mjCBase is virtual
+    object->model->DeleteElement(element);
+    return 0;
+  } catch (mjCError& e) {
+    object->model->SetError(e);
+    return -1;
+  }
 }