Added advice about not modifying the error parameter if a method does not directly return NO to indicate an error (Creating and Returning NSError Objects).

Initialized local variables in code samples.

Mentioned that NSError objects are available on iOS, but noted that the error-responder/recovery APIs and architecture are specific to the Mac. Also made some minor corrections and added links to core-competency concepts.

Corrected a link error.

Added link to "Error Handling in the Document Architecture" in Document-Based Application Overview. Mentioned default behavior of presentError: with NSCocoaErrorDomain/NSUserCancelledError errors. Provided related reference, sample code, and documents.

Corrected code in Listing 5-2 showing creation of NSInvocation object.

Corrected code listing illustrating error recovery and discussed NSUserCancelledError code.

New document that describes how to use NSError objects and related Application Kit support when handling user-level errors.