Handle JSONDecodeErrors in validator

[ml-evs]

Currently the validator doesn't provide any useful output (it crashes) when it encounters a malformed response that cannot be parsed as JSON. This PR catches that error and provides a more helpful error.

(see Materials-Consortia/providers#10 for context)

[codecov]

Codecov Report

Merging #144 into master will decrease coverage by 0.13%.
The diff coverage is 25%.

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
- Coverage   86.01%   85.87%   -0.14%     
==========================================
  Files          39       39              
  Lines        1830     1834       +4     
==========================================
+ Hits         1574     1575       +1     
- Misses        256      259       +3

Flag	Coverage Δ
#unittests	85.87% <25%> (-0.14%)	⬇️

Impacted Files	Coverage Δ
optimade/validator/validator.py	62.33% <25%> (-0.66%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update be7816b...df92b73. Read the comment docs.

[ml-evs]

We have our first eager deps failure!

Looks like lark v0.8 breaks all our tests, how fun.

[ltalirz]

Makes sense to me.

Hopefully the fastapi server won't actually return too many invalid jsons ;-)