Catch connection errors when populating client database list

[ml-evs]

Closes #1196.

[codecov]

Codecov Report

Merging #1197 (912461c) into master (05a0574) will decrease coverage by 0.04%.
The diff coverage is 50.00%.

❗ Current head 912461c differs from pull request most recent head d81a6c8. Consider uploading reports for the commit d81a6c8 to get more accurate results

@@            Coverage Diff             @@
##           master    #1197      +/-   ##
==========================================
- Coverage   91.80%   91.76%   -0.05%     
==========================================
  Files          72       72              
  Lines        4282     4285       +3     
==========================================
+ Hits         3931     3932       +1     
- Misses        351      353       +2     

Flag	Coverage Δ
project	91.76% <50.00%> (-0.05%)	⬇️
validator	91.15% <50.00%> (-0.05%)	⬇️

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Impacted Files	Coverage Δ
optimade/utils.py	95.23% <50.00%> (-3.10%)	⬇️

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