Preventing Core Data exceptions when no stores are attached

[bfahey]

Description

This PR prevents a crash which occurs when Core Data's persistent store fails to load. This can happen for several reasons: the database could be corrupted, the disk might be full, or from a bad migration. This also might occur when the app wakes up in the background and the persistent store didn't load in time.

Crash

The crash originates from IterableTaskScheduler.deleteAllTasks() which calls try persistenceContext.save() which throws:

NSInternalInconsistencyException - This NSPersistentStoreCoordinator has no persistent stores (disk full). It cannot perform a save operation.

          Fatal Exception: NSInternalInconsistencyException
0  CoreFoundation                 0x9cb4 __exceptionPreprocess
1  libobjc.A.dylib                0x183d0 objc_exception_throw
2  CoreData                       0x192388 -[NSPersistentStoreCoordinator _introspectLastErrorAndThrow]
3  CoreData                       0x19288c __65-[NSPersistentStoreCoordinator executeRequest:withContext:error:]_block_invoke.551
4  CoreData                       0x5f678 -[NSPersistentStoreCoordinator _routeHeavyweightBlock:]
5  CoreData                       0x256d8 -[NSPersistentStoreCoordinator executeRequest:withContext:error:]
6  CoreData                       0x256fc -[NSPersistentStoreCoordinator executeRequest:withContext:error:]
7  CoreData                       0x88220 -[NSManagedObjectContext save:]
8  Fetch Rewards                  0x1876084 closure #1 in IterableTaskScheduler.deleteAllTasks() + 177 (IterableCoreDataPersistence.swift:177)
9  Fetch Rewards                  0x18c3dcc thunk for @escaping @callee_guaranteed () -> () (<compiler-generated>)
10 CoreData                       0x80b6c developerSubmittedBlockToNSManagedObjectContextPerform
11 libdispatch.dylib              0x3eac _dispatch_client_callout
12 libdispatch.dylib              0xb534 _dispatch_lane_serial_drain
13 libdispatch.dylib              0xc0a4 _dispatch_lane_invoke
14 libdispatch.dylib              0x16cdc _dispatch_workloop_worker_thread
15 libsystem_pthread.dylib        0xddc _pthread_wqthread
16 libsystem_pthread.dylib        0xb7c start_wqthread

How to Fix

When using NSPersistentContainer it's important to handle any errors from loadPersistentStores: since they're recoverable. If you ignore this error, then Core Data throws an Objective-C exception which cannot be handled in Swift when you try to save.