API errors separate fast traffic growth (429 slow_down) from overload (503)

Updated API errors so applications can distinguish traffic that increases too quickly from temporary model overload.

Traffic that increases too quickly can return a 429 error with the slow_down code. Temporary model overload returns a 503 error with the server_is_overloaded code. Both responses may include Retry-After. When the header is present, wait at least as long as it specifies before retrying. If it's missing, use exponential backoff. See the error codes guide and rate limits guide.