Return a clearer response if no DB results
Return a clearer response if no DB results

This shouldn't happen, but it might, this will allow the underlying
error to be returned to the caller. The body of the response will now
look something like:

{
    "results": [],
    "error": "some error",
    "time": 0.021976516
}

rqlite/rqlite,rqlite/rqlite,rqlite/rqlite