Enhance error reporting in WASM codegen generators.

In `structs.py`, a `RuntimeError` is now raised when an anonymous struct is encountered but is not present in `STRUCTS_TO_BIND`.

PiperOrigin-RevId: 830978376
Change-Id: Id64720a12c3cce313bd513a40a74ddf57718a7d1

wasm/codegen/generators/structs.py

@@ -147,6 +147,12 @@ def _generate_field_data(
         anonymous_struct_name = name
         break
 
+    if not anonymous_struct_name:
+      raise RuntimeError(
+          f"Anonymous struct for field {f.name} in {w} not found in"
+          " ANONYMOUS_STRUCTS."
+      )
+
     if anonymous_struct_name in constants.STRUCTS_TO_BIND:
       return WrappedFieldData(
           binding=_simple_property_binding(f, w, setter=False, reference=True),