Multipart request body too large does not set error Status Code

### What happened?

If you upload a file that is too large via the Multipart transport, it returns an error to the client but does not set a non-200 error code.

### What did you expect?

I would expect to write a `http.StatusRequestEntityTooLarge` as the status.
This is the only error in the codebase I could find where we don't specifically set a status:

https://github.com/99designs/gqlgen/blob/d14fd791ab7c64788b041c6ace143e355aa37879/graphql/handler/transport/http_form_multipart.go#L64-L66