Remove check for MOC instance variable in `saveContextAndWait:..`

@mmorey 
This is very much up for discussion.

There seems to be an unnecessary check on `self.managedObjectContext` inside `saveContextAndWait:` .. as you previously mentioned this should never be `nil`

In an attempt to safeguard against it possibly being `nil` ... I've overridden `-(instanceType)init` to return nil in the event of `init` being accidentally called (instead of using one of the designated initializers)
Because this is not testing a condition an `ALog` would be a better choice.

```objective-c
- (instancetype)init {
    ALog(@"ERROR: Ensure MDMPersistenceController is instantiated using the designated initializer(s)");
    return nil;
}
```