Import: Check DB integrity

[Lavode]

If the DB file was copied while a transaction was ongoing, the resulting file will likely be corrupt.

To prevent wasting time trying to import such a corrupted file, it might make sense to check the DB's integrity using sqlite's built-in tools, before attempting the import.

Integrity can be checked by executing PRAGMA integrity_check. If the result is a single ok all is good, otherwise there's issues with the DB file.