Fixed problem with saving contexts that lead to segmentations faults.

When saving the contexts in `newValuesForObjectWithID:...` and `newValuesForRelationship:...`, it might happen under yet unclear conditions that the save operations don't finish successfully but don't leave any error behind. In consequence it will be attempted to dereference a NULL pointer to an error object within the subsequent `@throw` statement, leading to an EXC_BAD_ACCESS, code 2. I already described this bug [here](https://github.com/AFNetworking/AFIncrementalStore/issues/183), but couldn't come up with a solution until now.

This unwanted crash can be circumvented by generating a general CoreData error and assigning it to `error`, if `error` is NULL at a failed save request.