AFNetworking/AFNetworking — pull request #310 by cahn (2012-04-20 00:44:40+00:00)
Title: Try-catch in Objective-C should not be used to recover from exceptions

http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/Exceptions/Articles/ExceptionsAndCocoaFrameworks.html#//apple_ref/doc/uid/TP40009045-SW1

Relevant Quote:
"The Cocoa frameworks are generally not exception-safe. The general
pattern is that exceptions are reserved for programmer error only, and
the program catching such an exception should quit soon afterwards."

More discussion here:
http://stackoverflow.com/questions/324284/how-to-throw-an-exception-in-objective-c-cocoa

Given all this, prefer a crash on the background thread.