Fix Species validators and error messages

[ml-evs]

Closes #559

• No longer use length of dummy value
• Added code path for missing/invalid chemical symbols and validation of concentration
• Whitespace changes for legibility

[codecov]

Codecov Report

Merging #561 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #561   +/-   ##
=======================================
  Coverage   91.47%   91.48%           
=======================================
  Files          62       62           
  Lines        3110     3112    +2     
=======================================
+ Hits         2845     2847    +2     
  Misses        265      265           

Flag	Coverage Δ
#project	91.48% <100.00%> (+<0.01%)	⬆️
#validator	63.52% <60.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
optimade/models/structures.py	95.13% <100.00%> (+0.05%)	⬆️

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[ml-evs]

I made a minor tweak after review, switched from using pre=True to explicitly writing an error message for the case of missing/invalid chemical symbols. Going to take execute action and merge myself... A lot of this will be reworked in #560 anyway (although not for Species)