Provide feedback if trying to upload zero files

[dogversioning]

Now we'll provide a message about it. Addresses #352

Checklist

• Consider if documentation in docs/ needs to be updated
  • If you've changed the structure of a table, you may need to run generate-md
  • If you've added/removed core study fields that not in US Core, update our list of those in core-study-details.md
• Consider if tests should be added
• Update template repo if there are changes to study configuration in manifest.toml

[github-actions]

☂️ Python Coverage

current status: ✅

Overall Coverage

Lines	Covered	Coverage	Threshold	Status
2441	2441	100%	100%	🟢

New Files

No new covered files...

Modified Files

File	Coverage	Status
cumulus_library/actions/uploader.py	100%	🟢
TOTAL	100%	🟢

updated for commit: 5df9e98 by action🐍