Use fmt to write error message

[oyvindeide]

Approach
Short description of the approach

Pre review checklist

• Added appropriate labels

Adding labels helps the maintainers when writing release notes, see sections and the
corresponding labels here: https://github.com/equinor/ert/blob/main/.github/release.yml

[codecov-commenter]

Codecov Report

Merging #2974 (b9904a1) into main (324d702) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #2974      +/-   ##
==========================================
- Coverage   65.91%   65.91%   -0.01%     
==========================================
  Files         644      644              
  Lines       51540    51539       -1     
  Branches     4434     4433       -1     
==========================================
- Hits        33975    33973       -2     
+ Misses      16080    15932     -148     
- Partials     1485     1634     +149     

Impacted Files	Coverage Δ
libres/lib/analysis/update.cpp	27.91% <0.00%> (+0.08%)	⬆️
libres/lib/res_util/block_fs.cpp	53.67% <0.00%> (-0.30%)	⬇️
libres/lib/enkf/field.cpp	0.70% <0.00%> (ø)
libres/lib/config/conf.cpp	61.06% <0.00%> (ø)
libres/lib/enkf/gen_kw.cpp	59.61% <0.00%> (ø)
libres/lib/python/init.cpp	22.58% <0.00%> (ø)
libres/lib/enkf/surface.cpp	2.63% <0.00%> (ø)
libres/lib/rms/rms_file.cpp	0.00% <0.00%> (ø)
libres/lib/enkf/block_obs.cpp	39.25% <0.00%> (ø)
libres/lib/enkf/container.cpp	7.69% <0.00%> (ø)
... and 29 more

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