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

include/mujoco/mujoco.h

@@ -121,7 +121,7 @@ MJAPI void mj_freeLastXML(void);
 // If length of the output buffer is too small, returns the required size.
 MJAPI int mj_saveXMLString(const mjSpec* s, char* xml, int xml_sz, char* error, int error_sz);
 
-// Save spec to XML file, return 1 on success, 0 otherwise.
+// Save spec to XML file, return 0 on success, -1 otherwise.
 MJAPI int mj_saveXML(const mjSpec* s, const char* filename, char* error, int error_sz);
 
 
@@ -1453,8 +1453,8 @@ MJAPI mjsLight* mjs_addLight(mjsBody* body, const mjsDefault* def);
 // Add frame to body.
 MJAPI mjsFrame* mjs_addFrame(mjsBody* body, mjsFrame* parentframe);
 
-// Delete object corresponding to the given element.
-MJAPI void mjs_delete(mjsElement* element);
+// Delete object corresponding to the given element, return 0 on success.
+MJAPI int mjs_delete(mjsElement* element);
 
 
 //---------------------------------- Non-tree elements ---------------------------------------------