Add a test for GITErrorObjectNotFound to GITPackStoreTests.
Add a test for GITErrorObjectNotFound to GITPackStoreTests.

This now passes since 201815b3 where various segfaults with NSError
objects were causing crashes.

schacon/cocoagit,schacon/cocoagit,geoffgarside/cocoagit,geoffgarside/cocoagit