Add minor enhancements to compile binary.

* If output file exists, ask user whether to overwrite
* Fix erroneous check for whether XML file was successfully saved

PiperOrigin-RevId: 468743682
Change-Id: I06058c498328d333792e4b40d65c3a99ca1842e4

src/xml/xml_api.cc

@@ -120,6 +120,8 @@ mjModel* mj_loadXML(const char* filename, const mjVFS* vfs,
 
 
 // update XML data structures with info from low-level model, save as MJCF
+//  returns 1 if successful, 0 otherwise
+//  error can be NULL; otherwise assumed to have size error_sz
 int mj_saveLastXML(const char* filename, const mjModel* m, char* error, int error_sz) {
   // serialize access to themodel
   std::lock_guard<std::mutex> lock(themutex);