Use a dummy session variable.

Use a dummy session variable.

A little while ago I moved down the creation of the actual
DBSession() below the precondition checks to avoid unnecessary
rollback transactions. What I didn't do was check if the session
existed before using it, so when precondition checks failed I was
calling a variable that wasn't defined yet (another exception).
This adds a dummy session variable that I can check before rolling
back, avoiding more exceptions.