Visto API uses http status code to indicate success or failure of requests. Usually codes starting with number 2 means success, starting with number 4 means failure (e.g. Missing required parameter) and starting with number 5 means that there was a internal server error on Visto servers.

HTTP code error details
400 invalid_request Request HTTP type is invalid, request critical data is absent or malformed (e.g., no attachment body)
400 invalid_parameter Request parameter name or value is invalid
400 parameter_required Required parameter is absent
401 not_authorized User is not authorized (authorization is invalid, malformed or expired)
403 access_forbidden Access to requested entity is denied for user
403 not_allowed Requested action is not allowed due to license/quota limitations, e.t.c.
404 resource_not_found Requested entity is not found
404 method_not_found Requested API method does not exist
500 server_error Server side error