The NSURLErrorDomain error codes are listed here. However, 400 is just the http status code (http://www.w3.org/Protocols/HTTP/HTRESP.html) being returned which means you’ve got something wrong with your request.
The error has been documented on the Mac Developer Library(iOS docs) The concerned segment from the documentation will be: URL Loading System Error Codes These values are returned as the error code property of an NSError object with the domain “NSURLErrorDomain”. enum { NSURLErrorUnknown = -1, NSURLErrorCancelled = -999, NSURLErrorBadURL = -1000, NSURLErrorTimedOut = -1001, … Read more
All error codes are on “CFNetwork Errors Codes References” on the documentation (link) A small extraction for CFURL and CFURLConnection Errors: kCFURLErrorUnknown = -998, kCFURLErrorCancelled = -999, kCFURLErrorBadURL = -1000, kCFURLErrorTimedOut = -1001, kCFURLErrorUnsupportedURL = -1002, kCFURLErrorCannotFindHost = -1003, kCFURLErrorCannotConnectToHost = -1004, kCFURLErrorNetworkConnectionLost = -1005, kCFURLErrorDNSLookupFailed = -1006, kCFURLErrorHTTPTooManyRedirects = -1007, kCFURLErrorResourceUnavailable = -1008, kCFURLErrorNotConnectedToInternet … Read more