Replace assertion with an error message (backport #1834)

[mergify]

This is an automatic backport of pull request #1834 done by Mergify.

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[codecov]

Codecov Report

Merging #1836 (50bc3c5) into main (d3e311f) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

❗ Current head 50bc3c5 differs from pull request most recent head e34e150. Consider uploading reports for the commit e34e150 to get more accurate results

@@            Coverage Diff             @@
##             main    #1836      +/-   ##
==========================================
- Coverage   60.59%   60.58%   -0.01%     
==========================================
  Files          96       96              
  Lines       18860    18864       +4     
  Branches     9479     9483       +4     
==========================================
+ Hits        11428    11429       +1     
- Misses       5142     5146       +4     
+ Partials     2290     2289       -1     

Impacted Files	Coverage Δ
src/tiffcomposite_int.cpp	73.93% <0.00%> (-0.31%)	⬇️
src/jpgimage.cpp	70.15% <0.00%> (+0.13%)	⬆️

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