Add helpful CLI message to sleap-nn-train

[tom21100227]

Add helpful error message and more robust input handling to sleap-nn-train CLI:

Before:

After:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.30%. Comparing base (ff91433) to head (bac8c11).
⚠️ Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #294      +/-   ##
==========================================
+ Coverage   95.28%   95.30%   +0.02%     
==========================================
  Files          49       49              
  Lines        6765     6885     +120     
==========================================
+ Hits         6446     6562     +116     
- Misses        319      323       +4     

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[tom21100227]

• Check model.config instead of trainer.config.
• Check if there's a better solution than a stub for training.yaml