Mark mjCError and mjXError as [[nodiscard]].

This lets the compiler catch the mistake of creating an exception without throwing it.

PiperOrigin-RevId: 474536776
Change-Id: I5bd18258ce6ddd266adb1c852b1ba9e3063cd0b2

src/user/user_objects.h

@@ -84,7 +84,7 @@ typedef enum _mjtMeshType {
 
 
 // error information
-class mjCError {
+class [[nodiscard]] mjCError {
  public:
   mjCError(const mjCBase* obj = 0,
            const char* msg = 0,