Fix exit codes, date checks and knowledge base rebuild end date

[verovaleros]

Summary

Fixes problems found while reviewing #87 (development into main).

• rebuild-kb processed and cached today's incomplete data. The next day's blocklists would then miss attacks that arrived after the rebuild. The rebuild now stops at yesterday, and a start date of today is rejected.
• run-models with a start date after the end date did nothing and exited with success. It now prints an error and exits with code 1.
• Invalid dates printed an error but exited with code 0. They now exit with code 1, so scripts and scheduled jobs can detect the failure.
• A future date printed "Invalid date format". It now says the date cannot be in the future.
• generate_historical_blocklists.py imported lib.aip..., which fails with the documented PYTHONPATH=lib setup. It now imports aip... like the rest of the code.
• README: the rebuild command now says "until yesterday".

Testing

• Unit tests pass. The future-date test now also checks the error message.
• Checked by hand that a reversed date range, a badly formatted date, a future date and rebuild-kb for today each print an error and exit with code 1.
• generate_historical_blocklists.py imports without errors.