On master Grok Debugger returns all errors as 500, regardless of whether they originate as 4xx errors from ES: https://github.com/elastic/kibana/blob/master/x-pack/plugins/grokdebugger/server/routes/api/grokdebugger/register_grok_simulate_route.js#L40

We need to update its APIs to check if the error originated from ES and, if so, forward that information. Here's an example: https://github.com/elastic/kibana/blob/master/x-pack/plugins/index_management/server/routes/api/data_streams/register_get_route.ts#L60