Display error when loading model.

PiperOrigin-RevId: 830862449
Change-Id: Iab1e8786d79d55db9a15a788ecb37e0b0418111c

src/experimental/toolbox/helpers.cc

@@ -44,24 +44,20 @@ mjModel* LoadMujocoModel(const std::string& model_file, const mjVFS* vfs) {
   } else if (model_file.ends_with(".mjb")) {
     model = mj_loadModel(model_file.c_str(), 0);
     if (!model) {
-      mju_error("LoadMujocoModel mj_loadModel could not load file '%s'",
-                model_file.c_str());
+      return nullptr;
     }
   } else if (model_file.ends_with(".xml")) {
     char error[1000] = "";
     model = mj_loadXML(model_file.c_str(), vfs, error, sizeof(error));
     if (!model) {
-      mju_error("LoadMujocoModel mj_loadXML failed with '%s' for file '%s'",
-                error, model_file.c_str());
+      return nullptr;
     }
   } else {
     char error[1000] = "";
     auto spec =
         mj_parseXMLString(model_file.c_str(), nullptr, error, sizeof(error));
     if (!spec) {
-      mju_error(
-          "LoadMujocoModel mj_parseXMLString failed with '%s' for file '%s'",
-          error, model_file.c_str());
+      return nullptr;
     }
     model = mj_compile(spec, 0);
     mj_deleteSpec(spec);

src/experimental/toolbox/physics.cc

@@ -60,10 +60,17 @@ bool Physics::ProcessPendingLoad() {
   pending_load_.reset();
 
   model_ = LoadMujocoModel(model_file, vfs_);
-  if (!model_) mju_error("Error loading model");
+  if (!model_) {
+    error_ = "Error loading model!";
+    paused_ = true;
+    model_ = LoadMujocoModel("", vfs_);
+  }
 
   data_ = mj_makeData(model_);
-  if (!data_) mju_error("Error making data.");
+  if (!data_) {
+    error_ = "Error making data!";
+    paused_ = true;
+  }
 
   on_model_loaded_(model_file);