style: improve stack traces

[flying-sheep]

The EM lints prevent stack traces from having confusing almost-copies of the message next to the message.

https://docs.astral.sh/ruff/rules/raw-string-in-exception/

[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.20%. Comparing base (89df8df) to head (3e1324e).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/scverse_misc/_extensions.py	85.71%	2 Missing ⚠️
src/scverse_misc/datasets/_fetcher.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #88      +/-   ##
==========================================
- Coverage   93.36%   93.20%   -0.16%     
==========================================
  Files          12       12              
  Lines         708      721      +13     
==========================================
+ Hits          661      672      +11     
- Misses         47       49       +2     

Files with missing lines	Coverage Δ
src/scverse_misc/datasets/_registry.py	100.00% <100.00%> (ø)
src/scverse_misc/_extensions.py	84.26% <85.71%> (+0.12%)	⬆️
src/scverse_misc/datasets/_fetcher.py	96.66% <66.66%> (-1.58%)	⬇️

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