Some validation QoL tweaks

[ml-evs]

This PR addresses a few ergonomics/QoL concerns with the validator:

• Use single precision FP mach eps for elements_ratios validator (closes 'elements_ratios' model validator uses double-precision machine epsilon - could be relaxed #947)
• Use set operations in some of the chemical symbol field validators (might revert as it doesn't do quite what I wanted to, doubt there's any real speed boost) Dropped this for now.
• Tweak how 501 errors are handled in validator error messages - try to only give one error that uses the details from the implementation itself
• Simplify invalid chemical symbol error message by not printing the list of all elements.

[codecov]

Codecov Report

Merging #948 (5587980) into master (704fdd2) will increase coverage by 0.13%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master     #948      +/-   ##
==========================================
+ Coverage   92.78%   92.92%   +0.13%     
==========================================
  Files          67       67              
  Lines        3786     3787       +1     
==========================================
+ Hits         3513     3519       +6     
+ Misses        273      268       -5     

Flag	Coverage Δ
project	92.92% <90.90%> (+0.13%)	⬆️
validator	92.92% <90.90%> (+0.13%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
optimade/validator/validator.py	83.05% <85.71%> (+0.98%)	⬆️
optimade/models/structures.py	95.84% <100.00%> (-0.02%)	⬇️

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[JPBergsma]

Apart from the remark about the description of EPS the changes seem good to me.