Make sure all DB APIs return proper error message when the content requested is out of the pruner window.

[msmouse]

This is part of #103

1. Whenever possible, our error message should indicate that content is pruned.
2. When not possible (get_txn_by_hash), return None
3. APIs shouldn't panic on out-of-window queries
4. would be nice if we convert the DB interface to return AptDbError, so the tests can assert by error types. We can do something like impl From<anyhow::Error> for AptDbError and a AptDbError::Misc(anyhow::Error) so that not much needs to change.

[gregnazario]

Has this been cleaned up?

[msmouse]

I think we are in good shape with all the checks.