MTLManagedObjectAdapter: Fix a bug where errors are silently handled

It was using `error` but this error is replaced later in the method and never
passed as an error.

```objective-c
if (error != NULL) {
    *error = tmpError;
}
```

Mantle/MTLManagedObjectAdapter.m

@@ -392,7 +392,7 @@ - (id)managedObjectFromModel:(MTLModel<MTLManagedObjectSerializing> *)model inse
 			NSValueTransformer *attributeTransformer = [self entityAttributeTransformerForKey:propertyKey];
 			if (attributeTransformer != nil) transformedValue = [attributeTransformer transformedValue:transformedValue];
 
-			if (![managedObject validateValue:&transformedValue forKey:managedObjectKey error:error]) return NO;
+			if (![managedObject validateValue:&transformedValue forKey:managedObjectKey error:&tmpError]) return NO;
 			[managedObject setValue:transformedValue forKey:managedObjectKey];
 
 			return YES;