Migration 148: Fix drop table dependency order

The downgrade of 148 was broken because instance_actions_events has a FK
dependency on instance_actions but was attempted to be deleted first,
which caused the sqlalchemy to complain.

The fix is to reorder the table deletes in the downgrade.

Additional cleanups:

* Remove unecessary exception handling which masked errors

Change-Id: Iaac77ba0e81071f6bb39e5cc3b599a4138eb3cd2
Merge "Migration 148: Fix drop table dependency order"