print message to stderr when no valid results files are found

[atravitz]

currently, if you pass in an empty dir or mistype the your directory name, openfe gather will generate an empty table and even silently print it to -o.

I find this can be confusing and lead to unexpected behavior, so I'm adding a check up front, so that these scenarios are more transparent.

Checklist

• Added a news entry

Developers certificate of origin

• I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.69%. Comparing base (2b79724) to head (d30ee56).
Report is 164 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1245      +/-   ##
==========================================
- Coverage   93.40%   92.69%   -0.72%     
==========================================
  Files         163      141      -22     
  Lines       12206    10799    -1407     
==========================================
- Hits        11401    10010    -1391     
+ Misses        805      789      -16     

Flag	Coverage Δ
fast-tests	92.69% <100.00%> (-0.72%)	⬇️

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[github-actions]

No API break detected ✅