Improve invalid snapshot name recognition.

[joseph-roitman]

No description provided.

[codecov]

Codecov Report

Merging #43 (cc5f959) into master (72f5dc7) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #43   +/-   ##
=======================================
  Coverage   99.51%   99.51%           
=======================================
  Files           7        7           
  Lines         409      410    +1     
  Branches       46       47    +1     
=======================================
+ Hits          407      408    +1     
  Misses          2        2           

Impacted Files	Coverage Δ
tests/test_assert_match_dir.py	100.00% <ø> (ø)
tests/test_misc.py	100.00% <ø> (ø)
pytest_snapshot/plugin.py	100.00% <100.00%> (ø)

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