NSURLError（URL加载错误码）

/**
   https://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Miscellaneous/Foundation_Constants/index.html#//apple_ref/doc/constant_group/URL_Loading_System_Error_Codes

   NSURLErrorUnknown = -1,
   NSURLErrorCancelled = -999,
   NSURLErrorBadURL = -1000,
   NSURLErrorTimedOut = -1001,
   NSURLErrorUnsupportedURL = -1002,
   NSURLErrorCannotFindHost = -1003,
   NSURLErrorCannotConnectToHost = -1004,
   NSURLErrorDataLengthExceedsMaximum = -1103,
   NSURLErrorNetworkConnectionLost = -1005,
   NSURLErrorDNSLookupFailed = -1006,
   NSURLErrorHTTPTooManyRedirects = -1007,
   NSURLErrorResourceUnavailable = -1008,
   NSURLErrorNotConnectedToInternet = -1009,
   NSURLErrorRedirectToNonExistentLocation = -1010,
   NSURLErrorBadServerResponse = -1011,
   NSURLErrorUserCancelledAuthentication = -1012,
   NSURLErrorUserAuthenticationRequired = -1013,
   NSURLErrorZeroByteResource = -1014,
   NSURLErrorCannotDecodeRawData = -1015,
   NSURLErrorCannotDecodeContentData = -1016,
   NSURLErrorCannotParseResponse = -1017,
   NSURLErrorInternationalRoamingOff = -1018,
   NSURLErrorCallIsActive = -1019,
   NSURLErrorDataNotAllowed = -1020,
   NSURLErrorRequestBodyStreamExhausted = -1021,
   NSURLErrorFileDoesNotExist = -1100,
   NSURLErrorFileIsDirectory = -1101,
   NSURLErrorNoPermissionsToReadFile = -1102,
   NSURLErrorSecureConnectionFailed = -1200,
   NSURLErrorServerCertificateHasBadDate = -1201,
   NSURLErrorServerCertificateUntrusted = -1202,
   NSURLErrorServerCertificateHasUnknownRoot = -1203,
   NSURLErrorServerCertificateNotYetValid = -1204,
   NSURLErrorClientCertificateRejected = -1205,
   NSURLErrorClientCertificateRequired = -1206,
   NSURLErrorCannotLoadFromNetwork = -2000,
   NSURLErrorCannotCreateFile = -3000,
   NSURLErrorCannotOpenFile = -3001,
   NSURLErrorCannotCloseFile = -3002,
   NSURLErrorCannotWriteToFile = -3003,
   NSURLErrorCannotRemoveFile = -3004,
   NSURLErrorCannotMoveFile = -3005,
   NSURLErrorDownloadDecodingFailedMidStream = -3006,
   NSURLErrorDownloadDecodingFailedToComplete = -3007
 */