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;
}
```
MTLManagedObjectAdapter: Add tests for validating values
MTLManagedObjectAdapter: Fixes an issue with validate for insert

Fixes an issue where validation for insert would return a deleted
managed object without an error

When it does this in combination with
`relationshipModelClassesByPropertyKey` CoreData could raise an
exception for using setting child’s in a different context (because it
doesn’t have a context)
Merge pull request #220 from kylef/bug/managed-object-adapter/silent-error

MTLManagedObjectAdapter: Fix a bug where errors are silently handled