Report which body has too-small mass/inertia in compilation error.

PiperOrigin-RevId: 776196017
Change-Id: Ib81efb170347a57cdc4b446c14cd7b8f5357958c

test/user/user_mesh_test.cc

@@ -529,7 +529,7 @@ TEST_F(MjCMeshTest, TinyInertiaFails) {
       <mesh name="tiny" vertex="0 0 0  1e-4 0 0  0 1e-4 0  0 0 1e-4"/>
     </asset>
     <worldbody>
-      <body>
+      <body name="tiny_body">
         <freejoint/>
         <geom type="mesh" mesh="tiny"/>
       </body>
@@ -542,6 +542,7 @@ TEST_F(MjCMeshTest, TinyInertiaFails) {
       error.data(),
       HasSubstr(
           "mass and inertia of moving bodies must be larger than mjMINVAL"));
+  EXPECT_THAT(error.data(), HasSubstr("Element name 'tiny_body'"));
 }
 
 TEST_F(MjCMeshTest, FlippedFaceAllowedLegacyInertia) {

test/user/user_model_test.cc

@@ -196,7 +196,7 @@ TEST_F(UserCModelTest, NestedZeroMassBodiesFail) {
     <worldbody>
       <body>
         <geom size="1"/>
-        <body>
+        <body name="bad">
           <freejoint/>
           <body>
             <body>
@@ -214,6 +214,7 @@ TEST_F(UserCModelTest, NestedZeroMassBodiesFail) {
       error,
       HasSubstr(
           "mass and inertia of moving bodies must be larger than mjMINVAL"));
+  EXPECT_THAT(error, HasSubstr("Element name 'bad'"));
   mj_deleteModel(model);
 }