Описание тега nserror
An NSError object encapsulates richer and more extensible error information than is possible using only an error code or error string. The core attributes of an NSError object are an error domain (represented by a string), a domain-specific error code and a user info dictionary containing application specific information.
Several well-known domains are defined corresponding to Mach, POSIX, and OSStatus errors. Foundation error codes are found in the Cocoa error domain and documented in the Foundation Constants Reference. In addition, NSError allows you to attach an arbitrary user info dictionary to an error object, and provides the means to return a human-readable description for the error.
NSError is not an abstract class, and can be used directly. Applications may choose to create subclasses of NSError to provide better localized error strings by overriding -localizedDescription.
In general, a method should signal an error condition by—for example—returning NO or nil rather than by the simple presence of an error object. The method can then optionally return an NSError object by reference, in order to further describe the error.
NSError is toll-free bridged with its Core Foundation counterpart, CFError.