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,

src/xml/xml_urdf.cc

@@ -308,7 +308,7 @@ void mjXURDF::Joint(XMLElement* joint_elem) {
   ReadAttrTxt(joint_elem, "type", text, true);
   jointtype = FindKey(urJoint_map, urJoint_sz, text);
   if (jointtype < 0) {
-    mjXError(joint_elem, "invalid joint type in URDF joint definition");
+    throw mjXError(joint_elem, "invalid joint type in URDF joint definition");
   }
   ReadAttrTxt(joint_elem, "name", jntname, true);
 
@@ -317,7 +317,7 @@ void mjXURDF::Joint(XMLElement* joint_elem) {
   ReadAttrTxt(elem, "link", name, true);
   parent = (mjCBody*) model->GetWorld()->FindObject(mjOBJ_BODY, name);
   if (!parent) {                      // SHOULD NOT OCCUR
-    mjXError(elem, "invalid parent name in URDF joint definition");
+    throw mjXError(elem, "invalid parent name in URDF joint definition");
   }
 
   // get child=this, check

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,