Mojo C++ bindings: better log messages for some validation errors at the receiver side.

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 %}
 {%-     set kind = packed_field.field.kind %}
 {%-     if kind|is_object_kind %}
 {%-       set wrapper_type = kind|cpp_wrapper_type %}
 {%-       if not kind|is_nullable_kind %}
   if (!object->{{name}}.offset) {
     ReportValidationError(
-        mojo::internal::VALIDATION_ERROR_UNEXPECTED_NULL_POINTER);
+        mojo::internal::VALIDATION_ERROR_UNEXPECTED_NULL_POINTER,
+        "null {{name}} field in {{struct.name}} struct");
     return false;
   }
 {%-       endif %}
   if (!mojo::internal::ValidateEncodedPointer(&object->{{name}}.offset)) {
     ReportValidationError(mojo::internal::VALIDATION_ERROR_ILLEGAL_POINTER);
     return false;
   }
 {%-       if kind|is_any_array_kind or kind|is_string_kind %}
   if (!{{wrapper_type}}::Data_::Validate<
           {{kind|get_array_validate_params|indent(10)}}>(
               mojo::internal::DecodePointerRaw(&object->{{name}}.offset),
               bounds_checker)) {
 {%-       else %}
   if (!{{wrapper_type}}::Data_::Validate(
           mojo::internal::DecodePointerRaw(&object->{{name}}.offset),
           bounds_checker)) {
 {%-       endif %}
     return false;
   }
 {%-     elif kind|is_any_handle_kind %}
 {%-       if not kind|is_nullable_kind %}
   if (object->{{name}}.value() == mojo::internal::kEncodedInvalidHandleValue) {
     ReportValidationError(
-        mojo::internal::VALIDATION_ERROR_UNEXPECTED_INVALID_HANDLE);
+        mojo::internal::VALIDATION_ERROR_UNEXPECTED_INVALID_HANDLE,
+        "invalid {{name}} field in {{struct.name}} struct");
     return false;
   }
 {%-       endif %}
   if (!bounds_checker->ClaimHandle(object->{{name}})) {
     ReportValidationError(mojo::internal::VALIDATION_ERROR_ILLEGAL_HANDLE);
     return false;
   }
 {%-     endif %}
 {%-   endfor %}
 
@@ skipping 47 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_any_handle_kind %}
 mojo::internal::DecodeHandle(&{{pf.field.name}}, handles);
 {%-     endif %}
 {%-   endfor %}
 {%- endmacro -%}