[2-1-stable] use custom excpetions in Transactable

Caller can rescue only NestedSet excptions.

For example in controller:

<pre>
rescue_from CollectiveIdea::Acts::NestedSet::Model::Transactable::OpenTransactionsIsNotZero,
            CollectiveIdea::Acts::NestedSet::Model::Transactable::DeadlockDetected,
            :with => :nestedset_locked
</pre>