Send lox distributor error response as json

At the moment, the Lox distributor returns a Bad Request HTTP status code if there was an error with the request, along with the error string in the response body.

Many of the reasons for error are not due to bad client requests. Additionally, the json format will be easier to check and parse at the client. We can do something similar to the Snowflake broker messages where we return an error element only if it's present in the response.