Make error message consistent with GNU diff's implementation when failing to read input file(s)

[oSoMoN]

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.11765% with 4 lines in your changes missing coverage. Please review.

Project coverage is 75.52%. Comparing base (d362046) to head (8a3a977).

Files	Patch %	Lines
src/params.rs	95.00%	0 Missing and 2 partials :warning:
tests/integration.rs	83.33%	1 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #63      +/-   ##
==========================================
- Coverage   82.48%   75.52%   -6.97%     
==========================================
  Files          10       10              
  Lines        2935     2929       -6     
  Branches      744      746       +2     
==========================================
- Hits         2421     2212     -209     
+ Misses        495      490       -5     
- Partials       19      227     +208     

Flag	Coverage Δ
macos_latest	?
ubuntu_latest	75.92% <95.52%> (+0.47%)	:arrow_up:
windows_latest	15.87% <29.85%> (+0.45%)	:arrow_up:

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