388 slack fallback text

[TimJentzsch]

Relevant issue: Closes #388

Description:

This PR adds a fallback text for transcription check messages which is displayed in notifications.

Checklist:

• [x] Code Quality
• [x] Pep-8
• [x] Tests (if applicable)
• [x] Success Criteria Met
• [x] Inline Documentation
• [ ] Wiki Documentation (if applicable)