Log if backgroundContext was created in main thread

[Sorix]

Little addition to help developer to catch Core Data crashes if backgroundContext was created in incorrect thread. Very often Core Data in that cases shows in NSLog something non-human readable if you you’re accidentally forget about threads.

[3lvis]

Nothing worse than unwanted logs. I hate when that happens. Also is not a problem if you create a background context in the main thread, the problem is when you save a background context in the main thread. 💭