[core] Downgrade no item in trade error to warning

[WinterSolstice8]

I affirm:

• I understand that if I do not agree to the following points by completing the checkboxes my PR will be ignored.
• I understand I should leave resolving conversations to the LandSandBoat team so that reviewers won't miss what was said.
• I have read and understood the Contributing Guide and the Code of Conduct.
• I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

tl;dr: downgrading an error to a warning because it's spammy and isn't necessarily a cheat attempt like the ones after it.

Steps to test these changes

trigger condition, see warning not error