tor-dirmgr: Return an error if storage is readonly and DB is missing/incompatbile.

This fixes a bug in SqliteStore's constructor: previously, it would unconditionally try to create the missing database, even if it didn't have write access. As a result, it was impossible to reliably start multiple concurrent arti processes configured with the same (empty or nonexistent) cache_dir, because many of them would fail with errors such as

attempt to write a readonly database: Error code 8: Attempt to write a readonly database

Closes #1497