fix(#565): Handle OSError

[KyleKing]

Fixes: #565

Addresses edge case when incorrectly finding invalid files

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (56b24ef) to head (dd84e83).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #566   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines          956       961    +5     
  Branches       169       169           
=========================================
+ Hits           956       961    +5     

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[KyleKing]

@hukkin, this is ready for review

There was an unrelated Flake8 failure from CI, which I've addressed to pass all checks, but can be reverted or addressed with a flake8 ignore if you would prefer

 flake8...................................................................Failed
- hook id: flake8
- exit code: 1

src/mdformat/_cli.py:329:5: B042 Exception class with `__init__` should pass all args to `super().__init__()` to work in edge cases of `pickle` and `copy.copy()`. It should also not take any kwargs.