Don't throw exception if can't load entity

Not sure if a bug or a feature. Here's a scenario

1) User created a node with node-reference referencing nid 1, 2, 3
2) Other user deleted nid 1
3) Now, $wrapper->field_node_ref->value() will throw an exception.

Since there's no field API bulk delete, we can't make sure the field storage has the most up-to-date data.