For NSSavePanel delegates, asks the delegate for file URL validation when the user chooses the Save button. For NSOpenPanel delegates, asks the delegate for file URL validation once for each selected filename (or directory) when the user chooses the Open button.
The panel requesting URL validation.
The URL to be validated.
If an error occurred during validation, the error that occurred.
YES if the URL is an acceptable URL to save to or to open; otherwise, NO.