Add key name to "Value not set" error message

[jim-easterbrook]

The error typically occurs when writeMetadata() is called. It's easier
to find the problem if you know which datum hasn't had a value set.

[codecov]

Codecov Report

Merging #2149 (04904db) into main (173ebb2) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main    #2149   +/-   ##
=======================================
  Coverage   63.31%   63.31%           
=======================================
  Files          97       97           
  Lines       19152    19152           
  Branches     9724     9724           
=======================================
  Hits        12127    12127           
  Misses       4762     4762           
  Partials     2263     2263           

Impacted Files	Coverage Δ
src/error.cpp	83.87% <ø> (ø)
src/exif.cpp	63.66% <0.00%> (ø)
src/iptc.cpp	66.10% <0.00%> (ø)
src/xmp.cpp	77.33% <0.00%> (ø)

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[piponazo]

Hi @jim-easterbrook , thanks for this contribution. I think the change you are proposing here makes sense.

Could you please rebase your branch in top of main ? We just merged some changes which slightly change the Error class.

[jim-easterbrook]

Done, and still working OK.

[piponazo]

LGTM! Thanks!