mojo::Serialize*_() calls now propogate/return validation errors.

mojo/public/tools/bindings/generators/cpp_templates/struct_serialization_definition.tmpl

 {%- import "struct_macros.tmpl" as struct_macros %}
 size_t GetSerializedSize_(const {{struct.name}}& input) {
   {{struct_macros.get_serialized_size(struct, "input.%s")}}
   return size;
 }
 
-void Serialize_({{struct.name}}* input, mojo::internal::Buffer* buf,
-                internal::{{struct.name}}_Data** output) {
+mojo::internal::ValidationError Serialize_(
+    {{struct.name}}* input,
+    mojo::internal::Buffer* buf,
+    internal::{{struct.name}}_Data** output) {
   if (input) {
-    {{struct_macros.serialize(struct, struct.name ~ " struct", "input->%s", "result", "buf")|indent(2)}}
+    {{struct_macros.serialize(struct, struct.name ~ " struct", "input->%s", "result", "buf", true)|indent(2)}}
     *output = result;
   } else {
     *output = nullptr;
   }
+  return mojo::internal::VALIDATION_ERROR_NONE;
 }
 
 void Deserialize_(internal::{{struct.name}}_Data* input,
                   {{struct.name}}* result) {
   if (input) {
     {{struct_macros.deserialize(struct, "input", "result->%s")|indent(2)}}
   }
 }