Provide better user feedback for Zoekt search syntax errors

Currently we just return 500 when a user performs a Zoekt search with invalid Zoekt syntax. We should improve the UX to at least show an empty search results page. We may be able to gather slightly more detail and tell them it was due to a search syntax problem. It's possible that Zoekt doesn't give us enough detail in the error to say if it's a syntax error but at the least we should provide a generic enough message like "invalid search" so they don't think it's an intermittent server error.