Remove unused import
Don't include exception info when no exception was thrown
Update usage of renamed variable
Set active transaction explicitly, don't use newest
Let the client override active transaction, if given id exists
Set active for currently existing transaction objects

If we'd skip this, currently ongoing enrollements might get errors since
the get_active_transaction() implementation has changed, and relies on
the underlying datamodel to be updated.
Merge branch 'active-transaction'