improve pkgchk_has_vignette

[mpadge]

Turns out xfun::taml_load can error from recursive indexing. This catches any such errors, and manually processes the taml header in such cases to approximately grep the output format

[codecov]

Codecov Report

❌ Patch coverage is 23.52941% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.27%. Comparing base (ace0726) to head (c82644d).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
R/check-vignette.R	23.52%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #315      +/-   ##
==========================================
- Coverage   92.54%   92.27%   -0.28%     
==========================================
  Files          47       47              
  Lines        3569     3585      +16     
==========================================
+ Hits         3303     3308       +5     
- Misses        266      277      +11     

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