Add macro to err strings

[benjeffery]

Fixes #2264

[benjeffery]

Rebased and ready to go.

[codecov]

Codecov Report

Merging #2268 (d832d01) into main (23c8a24) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2268   +/-   ##
=======================================
  Coverage   93.29%   93.29%           
=======================================
  Files          27       27           
  Lines       26089    26089           
  Branches     1172     1172           
=======================================
  Hits        24341    24341           
  Misses       1718     1718           
  Partials       30       30           

Flag	Coverage Δ
c-tests	92.23% <100.00%> (ø)
lwt-tests	89.05% <ø> (ø)
python-c-tests	71.90% <ø> (ø)
python-tests	98.88% <ø> (ø)

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Impacted Files	Coverage Δ
c/tskit/core.c	97.88% <100.00%> (ø)

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