NSUserCancelledError = 3072, // User cancelled operation (this one often doesn't deserve a panel and might be a good one to special case)
If you cancel an outstanding request (via the cancel method on the NSProgress object, or cancelling a parent progress object you have created) the completion handler argument to this method will be called back with an NSUserCancelledError in the NSCocoaErrorDomain.
/* Cancel all invocations of -coordinate... and -prepare... methods for the receiver. Any current invocation of one of those methods will stop waiting and return immediately, unless it has already invoked the passed-in block, in which case it will return when the passed-in block returns. Subsequent invocations of those methods will not invoke the blocks passed into them at all. When an invocation of -coordinate... or -prepare... returns without invoking the passed-in block because this method was invoked it instead returns an error whose domai