Exception.rb : class TransactionError was inserted

I added the Class TransactionError in Exception.rb so now I can do this:

```ruby
if @lecture.valid?
    begin
        Maglev.commit_transaction
    rescue TransactionError => error
        puts error.object.class
        puts error.object
        raise
   end
end
```

When a TransactionError is raised I want to know more about the object that was transient than just the class name.