[Unity plugin] Surface a clear error when mesh asset is missing

When an MJCF references a mesh whose asset failed to import, or whose
name collides with another asset of a different type in the Resources
folder, Resources.Load<Mesh>(...) returns null. The Mesh field is then
dereferenced silently later (in BuildMesh / DebugDraw), producing an
opaque NullReferenceException far from the root cause.

Throw a descriptive exception at the load site so users hit the import
abort path with a message that points at the actual problem.

Related to #1354 (typed Resources.Load was already in place; this adds
the missing diagnostic on top).

Author: ingyukoh

unity/Runtime/Components/Shapes/MjMeshShape.cs

@@ -37,6 +37,12 @@ public void FromMjcf(XmlElement mjcf) {
         mjcf.GetStringAttribute("mesh", defaultValue: string.Empty));
     if (!string.IsNullOrEmpty(assetName)) {
       Mesh = Resources.Load<Mesh>(assetName);
+      if (Mesh == null) {
+        throw new Exception(
+            $"Could not load mesh asset '{assetName}'. The MJCF references a mesh "
+            + "that was not imported, or another asset of a different type (e.g. a "
+            + "material) shares the same name in a Resources folder.");
+      }
     }
   }