Give warning instead of error for wrong mongo URI

[JPBergsma]

Quick fix for issue #2197 By raising a warning instead of an error optimade should still work even when env vars are used,
and always taking the mongo_database value from the Uri.

I leave it up to @ml-evs to make a more permanent solution.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.69%. Comparing base (9af05bc) to head (ba5a9ad).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2198      +/-   ##
==========================================
+ Coverage   90.65%   90.69%   +0.04%     
==========================================
  Files          75       75              
  Lines        4858     4858              
==========================================
+ Hits         4404     4406       +2     
+ Misses        454      452       -2     

Flag	Coverage Δ
project	90.69% <100.00%> (+0.04%)	⬆️
validator	90.69% <100.00%> (+0.04%)	⬆️

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[ml-evs]

Thanks for this @JPBergsma!