MasteringOpenCV/code — pull request #32 by SSteve (2014-03-22 06:47:09+00:00)
Title: Check that the return value is nil before attempting to do anything with...

... the NSError object

Apple's Error Handling Programming Guide
(https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/
ErrorHandlingCocoa/CreateCustomizeNSError/CreateCustomizeNSError.html)
states that return values must be checked for nil or NO before
attempting to do anything with the NSError object