iOS app crashes with NSInvalidArgumentException when a cloning error is generated

The easiest way I have found to reproduce this exception is to try and clone a repository to a directory that already exists:

```objective-c
NSURL *url = [NSURL URLWithString:@"http://github.com/libgit2/objective-git.git"];
NSURL *dir = [NSURL fileURLWithPath:NSHomeDirectory()];
NSError *error = nil;
[GTRepository cloneFromURL:url toWorkingDirectory:dir barely:YES withCheckout:NO error:&error transferProgressBlock:NULL checkoutProgressBlock:NULL];
NSLog(@"%@", error);
```

This produces the following exception:
```
+[NSError git_errorFor:withAdditionalDescription:]: unrecognized selector sent to class 0x163d26c
Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '+[NSError git_errorFor:withAdditionalDescription:]: unrecognized selector sent to class 0x163d26c'
```

Here is the call stack:

```
*** First throw call stack:
(
	0   CoreFoundation                      0x019eb6f4 __exceptionPreprocess + 180
	1   libobjc.A.dylib                     0x0176b8b6 objc_exception_throw + 44
	2   CoreFoundation                      0x01a88823 +[NSObject(NSObject) doesNotRecognizeSelector:] + 275
	3   CoreFoundation                      0x019dba1b ___forwarding___ + 1019
	4   CoreFoundation                      0x019db5fe _CF_forwarding_prep_0 + 14
	5   CloneiOS                            0x000081f6 +[GTRepository cloneFromURL:toWorkingDirectory:barely:withCheckout:error:transferProgressBlock:checkoutProgressBlock:] + 1398
	6   CloneiOS                            0x00005fa3 -[AppDelegate application:didFinishLaunchingWithOptions:] + 339
	7   UIKit                               0x004dd525 -[UIApplication _handleDelegateCallbacksWithOptions:isSuspended:restoreState:] + 309
	8   UIKit                               0x004ddd65 -[UIApplication _callInitializationDelegatesForURL:payload:suspended:] + 1536
	9   UIKit                               0x004e2578 -[UIApplication _runWithURL:payload:launchOrientation:statusBarStyle:statusBarHidden:] + 824
	10  UIKit                               0x004f657c -[UIApplication handleEvent:withNewEvent:] + 3447
	11  UIKit                               0x004f6ae9 -[UIApplication sendEvent:] + 85
	12  UIKit                               0x004e41f5 _UIApplicationHandleEvent + 736
	13  GraphicsServices                    0x038f633b _PurpleEventCallback + 776
	14  GraphicsServices                    0x038f5e46 PurpleEventCallback + 46
	15  CoreFoundation                      0x01966e95 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 53
	16  CoreFoundation                      0x01966bcb __CFRunLoopDoSource1 + 523
	17  CoreFoundation                      0x019918ac __CFRunLoopRun + 2156
	18  CoreFoundation                      0x01990bf3 CFRunLoopRunSpecific + 467
	19  CoreFoundation                      0x01990a0b CFRunLoopRunInMode + 123
	20  UIKit                               0x004e1cad -[UIApplication _run] + 840
	21  UIKit                               0x004e3f0b UIApplicationMain + 1225
	22  CloneiOS                            0x0000656d main + 141
	23  libdyld.dylib                       0x01f2a725 start + 0
)
libc++abi.dylib: terminating with uncaught exception of type NSException
```

When I run the same code in a Mac OS X app it returns an error as expected and the app doesn't crash.