Don't assert if the managed object model can't be initialized. Not sure why...

Don't assert if the managed object model can't be initialized. Not sure why this would happen (bundle corruption?), but it's a pointless crash.

sources/iTermShellHistoryController.m

@@ -8,6 +8,7 @@
 #import "iTermShellHistoryController.h"
+#import "DebugLogging.h"
 #import "iTermCommandHistoryEntryMO+Additions.h"
 #import "iTermDirectoryTree.h"
 #import "iTermHostRecordMO.h"
@@ -162,7 +163,10 @@ static const NSTimeInterval kMaxTimeToRememberDirectories = 60 * 60 * 24 * 90;
     NSManagedObjectModel *managedObjectModel =
         [[[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL] autorelease];
-    assert(managedObjectModel);
+    if (!managedObjectModel) {
+        ELog(@"Failed to initialize managed object model for URL %@", modelURL);
+        return NO;
+    }
     NSPersistentStoreCoordinator *persistentStoreCoordinator =
         [[[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:managedObjectModel] autorelease];