Improve JSON encoding error message

[benjeffery]

Fixes #1505

[codecov]

Codecov Report

Merging #1827 (c76fbd6) into main (5bd854d) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head c76fbd6 differs from pull request most recent head 647d36f. Consider uploading reports for the commit 647d36f to get more accurate results

@@            Coverage Diff             @@
##             main    #1827      +/-   ##
==========================================
- Coverage   93.39%   93.38%   -0.01%     
==========================================
  Files          27       27              
  Lines       24814    24787      -27     
  Branches     1094     1094              
==========================================
- Hits        23175    23148      -27     
  Misses       1604     1604              
  Partials       35       35              

Flag	Coverage Δ
c-tests	92.11% <ø> (-0.02%)	⬇️
lwt-tests	89.14% <ø> (ø)
python-c-tests	94.53% <100.00%> (-0.01%)	⬇️
python-tests	98.75% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
python/tskit/exceptions.py	100.00% <100.00%> (ø)
python/tskit/metadata.py	98.85% <100.00%> (+<0.01%)	⬆️
c/tskit/trees.c	94.89% <0.00%> (-0.04%)	⬇️
python/_tskitmodule.c	91.67% <0.00%> (-0.01%)	⬇️
python/tskit/trees.py	97.83% <0.00%> (-0.01%)	⬇️

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[jeromekelleher]

LGTM