Fix error message for merchants with declined status

[ksere]

Changes proposed in this Pull Request:

Closes #230

Fixes the expected error message for merchants with declined status:
From: "Could not fetch account status" to "Your merchant account is disapproved."

Screenshots:

Detailed test instructions:

1. Start onboarding and connect to a new account.
2. Wait till it gets the "Declined" status from Pinterest
3. Verify the correct error message is shown in the plugin's settings.

Changelog entry

Fix - Wrong error message on declined Pinterest account.