With every request you'll get these responses:

200OKThe request has succeeded.
202AcceptedAPI call has been accepted for processing. This code will be returned by for asynchronous operations (Batches).
204No contentThe server successfully processed the request, but is not returning any content
400Bad Requestinvalid parameters, fields or filters
401Unauthorizedauthorization error (invalid basic auth data or API key)
403ForbiddenThe server understood the request, but is refusing it (blocked due to many errors in particular time) or the access is not allowed.
404Not foundThere is no resource behind the URI.
406Not acceptableResource or method is not available or was removed.
408Timeoutrequest timeout
422Unprocessable Entityused if the server cannot process the enitity, e.g. mandatory fields are missing in the payload.
429Too many requestsRate limit exceeded.
503Internal server errorWe were unable to handle the HTTP request due to a temporary overloading or maintenance of the server. Please try again later.

With 4xx response (error) you'll receive in response body error with error description and fields array with all errors.

"error":"Bad parameter: cartID not found.",
  "'status' can't be 'unknown'. Available values: subscribed,nonsubscribed,nonSubscribed,unsubscribed"