ci: make curl fail on unsuccessful call to slack API

[barmac]

Closes #4417

Proposed Changes

This PR makes us use curl --fail-with-body so that when API call fails, the action fails too.

Cf. https://superuser.com/questions/590099/can-i-make-curl-fail-with-an-exitcode-different-than-0-if-the-http-status-code-i

Checklist

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• Brief textual description of the changes present
• Visual demo attached
• Steps to try out present, i.e. using the @bpmn-io/sr tool
• Related issue linked via Closes {LINK_TO_ISSUE} or Related to {LINK_TO_ISSUE}

[marstamm]

let's try it out