Improve semver validation error messsage

[ml-evs]

Minor fix arising from some validation of AFLOW yesterday (that "1.0" is not a valid semver, and also that semver may not be well known as an abbreviation).

[codecov]

Codecov Report

Merging #845 (22f6d55) into master (97c0c89) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #845   +/-   ##
=======================================
  Coverage   92.70%   92.70%           
=======================================
  Files          67       67           
  Lines        3699     3699           
=======================================
  Hits         3429     3429           
  Misses        270      270           

Flag	Coverage Δ
project	92.70% <100.00%> (ø)
validator	92.70% <100.00%> (ø)

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Impacted Files	Coverage Δ
optimade/models/utils.py	91.56% <100.00%> (ø)

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[CasperWA]

Cheers @ml-evs !