👌 Improve directive parsing warnings

[chrisjsewell]

Minor improvement to the specificity of warnings regarding problems encountered when parsing directives

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 80.00000% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 90.06%. Comparing base (978e845) to head (ee4ae60).

Files	Patch %	Lines
myst_parser/parsers/directives.py	81.25%	3 Missing ⚠️
myst_parser/mocking.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #893      +/-   ##
==========================================
+ Coverage   90.04%   90.06%   +0.02%     
==========================================
  Files          24       24              
  Lines        3364     3372       +8     
==========================================
+ Hits         3029     3037       +8     
  Misses        335      335              

Flag	Coverage Δ
pytests	90.06% <80.00%> (+0.02%)	⬆️

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