Improve some validator error messages

[ml-evs]

This PR makes a couple of minor tweaks to the validator.

• Prevent internal failures when checking meta->schema of a request that has already failed
• Improve a couple of error messages

[codecov]

Codecov Report

Merging #1557 (e8cbd05) into master (20b1e87) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1557   +/-   ##
=======================================
  Coverage   91.07%   91.07%           
=======================================
  Files          74       74           
  Lines        4538     4538           
=======================================
  Hits         4133     4133           
  Misses        405      405           

Flag	Coverage Δ
project	91.07% <100.00%> (ø)
validator	90.96% <100.00%> (ø)

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

Impacted Files	Coverage Δ
optimade/validator/validator.py	83.51% <100.00%> (ø)

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