Describe the bug
In Nix 2.3 you might see a warning: SQLite database '/nix/var/nix/db/db.sqlite' is busy when the database is busy. But in Nix >=2.8 (and possibly earlier?) you get a more confusing message:

warning: error: SQLite database '/nix/var/nix/db/db.sqlite' is busy

I've been asked about this more than once at $dayjob: "Is this message a warning or an error?". I'm pretty sure it's just a warning, and if that's correct, we should probably remove the error: part from that message.

Steps To Reproduce

1. Run many build actions at once.
2. See warning. (Or error :-) )

Expected behavior

I expect messages to be either warning or error, not both.