Add a clearer error message on when trying to use client with missing deps

[ml-evs]

This PR simply adds an import guard around rich and httpx with a more user-friendly error message.

[codecov]

Codecov Report

Merging #1200 (3e5c9cb) into master (6c2e768) will decrease coverage by 0.04%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1200      +/-   ##
==========================================
- Coverage   91.84%   91.80%   -0.05%     
==========================================
  Files          72       72              
  Lines        4279     4282       +3     
==========================================
+ Hits         3930     3931       +1     
- Misses        349      351       +2     

Flag	Coverage Δ
project	91.80% <66.66%> (-0.05%)	⬇️
validator	91.19% <66.66%> (-0.05%)	⬇️

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Impacted Files	Coverage Δ
optimade/client/client.py	78.90% <66.66%> (-0.59%)	⬇️

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