Mojo cpp bindings: report the reason of validation failure.

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

 {%- macro validate(struct, class_name) %}
   if (!data)
     return true;
 
   if (!ValidateStructHeader(
           data, sizeof({{class_name}}),
           {{struct.packed.packed_fields|length}}, bounds_checker)) {
     return false;
   }
 
   const {{class_name}}* MOJO_ALLOW_UNUSED object =
       static_cast<const {{class_name}}*>(data);
 
 {%-   for packed_field in struct.packed.packed_fields %}
 {%-     set name = packed_field.field.name %}
 {%-     if packed_field.field.kind|is_object_kind %}
 {%-       set wrapper_type = packed_field.field.kind|cpp_wrapper_type %}
-  if (!mojo::internal::ValidateEncodedPointer(&object->{{name}}.offset) ||
-      !{{wrapper_type}}::Data_::Validate(
+  if (!mojo::internal::ValidateEncodedPointer(&object->{{name}}.offset)) {
+    ReportValidationError(mojo::internal::VALIDATION_ERROR_ILLEGAL_POINTER);
+    return false;
+  }
+  if (!{{wrapper_type}}::Data_::Validate(
           mojo::internal::DecodePointerRaw(&object->{{name}}.offset),
           bounds_checker)) {
     return false;
   }
 {%-     elif packed_field.field.kind|is_handle_kind %}
-  if (!bounds_checker->ClaimHandle(object->{{name}}))
+  if (!bounds_checker->ClaimHandle(object->{{name}})) {
+    ReportValidationError(mojo::internal::VALIDATION_ERROR_ILLEGAL_HANDLE);
     return false;
+  }
 {%-     endif %}
 {%-   endfor %}
 
   return true;
 {%- endmacro %}
 
 {%- macro field_line(field) %}
 {%-   set type = field.kind|cpp_field_type %}
 {%-   set name = field.name -%}
 {%-   if field.kind.spec == 'b' -%}
@@ skipping 40 matching lines @@
 
 {%- macro decodes(struct) -%}
 {%-   for pf in struct.packed.packed_fields %}
 {%-     if pf.field.kind|is_object_kind %}
 mojo::internal::Decode(&{{pf.field.name}}, handles);
 {%-     elif pf.field.kind|is_handle_kind %}
 mojo::internal::DecodeHandle(&{{pf.field.name}}, handles);
 {%-     endif %}
 {%-   endfor %}
 {%- endmacro -%}