Update Migrator to expect DataStoreException

Migrator was still expecting a DbException when history table doesn't exist. But this is now a DataStoreException.

Also updating the relevant unit test. Note the unit test didn't catch this change. Catching this change would require adding functional tests that hit the actual data store exception when the table doesn't exist. There aren't any functional tests for migrations as yet.
The table not existing is one scenario but is that the only exception that can be thrown? Do you want to catch both DataStoreException and DbException?