Clean up error handling, and report a better error message
on search and flush if FTS5 tables haven't yet been created.
Add some mypy comments to clean up type-checking errors.
Use SQLite's FTS5 extension to power full-text search without any
additional dependencies. FTS5 was introduced in SQLite 3.9.0,
[released][1] in 2015 so should be available on most SQLite
installations at this point in time.
[1]: https://www.sqlite.org/changes.html#version_3_9_0
