Print url when download fails

[grahamgower]

Closes #697.

[codecov]

Codecov Report

Merging #1285 (117e9d7) into main (f782e49) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1285   +/-   ##
=======================================
  Coverage   99.66%   99.66%           
=======================================
  Files         111      111           
  Lines        3559     3563    +4     
  Branches      438      438           
=======================================
+ Hits         3547     3551    +4     
  Misses          8        8           
  Partials        4        4           

Impacted Files	Coverage Δ
stdpopsim/utils.py	100.00% <100.00%> (ø)

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[jeromekelleher]

LGTM