Improved error message for bad version returning 553

[ml-evs]

Closes #493

[codecov]

Codecov Report

Merging #494 into master will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #494   +/-   ##
=======================================
  Coverage   91.48%   91.48%           
=======================================
  Files          61       61           
  Lines        3065     3065           
=======================================
  Hits         2804     2804           
  Misses        261      261           

Flag	Coverage Δ
#project	91.48% <0.00%> (ø)
#validator	63.29% <0.00%> (ø)

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Impacted Files	Coverage Δ
optimade/validator/validator.py	82.18% <0.00%> (ø)

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[CasperWA]

Looks fine to me, thanks @ml-evs !