Fix confusing Slack credential missing error
This error message does not make sense. Obviously the newly subscribed client does not have any Slack token. Fix it

Outcome
Signup card's Slack failure now names jaah's own setting ("no Slack credential is set in Settings > Integrations > Slack") instead of the misleading slack-credential-missing. Tests + quick gate green; PR #6706 queued to merge lane. Memory: none applied.
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