Add action to exit non-zero if file need changes/changed.

[Carreau]

This seem ti was a bug; but the default seem to be to always return 0, maybe should be changed ?

[codecov]

Codecov Report

❌ Patch coverage is 18.18182% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.30%. Comparing base (41733ec) to head (2f058bd).
⚠️ Report is 224 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/python/pyflyby/_cmdline.py	10.00%	9 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (41733ec) and HEAD (2f058bd). Click for more details.

HEAD has 3 uploads less than BASE

Flag	BASE (41733ec)	HEAD (2f058bd)
	10	7

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #463       +/-   ##
===========================================
- Coverage   67.97%   56.30%   -11.68%     
===========================================
  Files          25       29        +4     
  Lines        8012     8751      +739     
===========================================
- Hits         5446     4927      -519     
- Misses       2566     3824     +1258     

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[Carreau]

consensus was to not change the default and keep this as-is