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

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

Index: mojo/public/tools/bindings/generators/cpp_templates/struct_macros.tmpl
diff --git a/mojo/public/tools/bindings/generators/cpp_templates/struct_macros.tmpl b/mojo/public/tools/bindings/generators/cpp_templates/struct_macros.tmpl
index 5ce9268e5f02abe14adde9915b57ea99b83b474a..0e5119323996ee8044a009aaa3eaaccd1c5fb21b 100644
--- a/mojo/public/tools/bindings/generators/cpp_templates/struct_macros.tmpl
+++ b/mojo/public/tools/bindings/generators/cpp_templates/struct_macros.tmpl
@@ -19,7 +19,8 @@
 {%-       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 %}
@@ -43,7 +44,8 @@
 {%-       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 %}