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/xml/xml_util.h

@@ -33,7 +33,7 @@ void mjCopyError(char* dst, const char* src, int maxlen);
 
 
 // XML Error info
-class mjXError {
+class [[nodiscard]] mjXError {
  public:
   mjXError(const tinyxml2::XMLElement* elem = 0,
            const char* msg = 0,