document error response

[aligthart]

The API documentation does not state anything on the format of errors that can be returned when doing calls to gubernator.

On an error you can still get an http 200 back but a totally different response. (for instance the one below during the restart of a ratelimiter POD).

Would be good to document the possible error formats.

{"responses":[
    {"error":"while finding peer that owns rate limit 'xxxx' - 'unable to pick a peer; pool is empty'"},
    {"error":"while finding peer that owns rate limit 'yyyy' - 'unable to pick a peer; pool is empty'"}
]}