Improving errors for DNS name parsing

[ctz]

(exploratory currently, opened just for CI)

[codecov]

Codecov Report

❌ Patch coverage is 94.33962% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.84%. Comparing base (05be248) to head (c2ce8a1).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/subject_name/dns_name.rs	94.11%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #533      +/-   ##
==========================================
+ Coverage   97.63%   97.84%   +0.21%     
==========================================
  Files          20       20              
  Lines        4099     4135      +36     
==========================================
+ Hits         4002     4046      +44     
+ Misses         97       89       -8     

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[ctz]

(having slept on it, I am instead planning on putting this information into InvalidNameContext, so each name in there also comes with an optional error expressing why it wasn't considered. I think that will produce a more useful error.)