make error messages runable

[bart1]

see https://cli.r-lib.org/reference/links.html?q=.run#click-to-run-code for details

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.72%. Comparing base (b28e9d7) to head (92ff1ef).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
R/get_pvol_nl.R	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #212      +/-   ##
==========================================
- Coverage   93.44%   91.72%   -1.73%     
==========================================
  Files          30       31       +1     
  Lines        2229     2272      +43     
==========================================
+ Hits         2083     2084       +1     
- Misses        146      188      +42     

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