translateDomainError only recognized web.HTTPErrorProcessor, so a
ValidationHTTPError from InvalidFieldError (e.g. an unknown webhook
event) leaked as a 500 instead of the 412 v1 returns. It carries the
status via GetHTTPCode() but cannot implement HTTPErrorProcessor because
the embedded web.HTTPError field shadows the method name. Add a
GetHTTPCode/GetCode branch so v2 surfaces the right status and preserves
the v1 numeric code on the body.