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.
We should always throw that exception (that was the work I did last week to make sure we throw a DataStoreException when a query/save operation fails). It's definitely imperfect, but the purpose of this catch block is to detect that the table doesn't exist.