Capture `ConverterError`s in molecules_from_xyz()

[alongd]

The set_multiplicity() function can raise a ConverterError which should also be captured instead of crashing ARC

[codecov[bot]]

Codecov Report

Merging #615 (711afa9) into main (1e992b0) will increase coverage by 0.00%. The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main     #615   +/-   ##
=======================================
  Coverage   73.02%   73.03%           
=======================================
  Files          99       99           
  Lines       26178    26178           
  Branches     5482     5482           
=======================================
+ Hits        19116    19118    +2     
+ Misses       5699     5698    -1     
+ Partials     1363     1362    -1     

Impacted Files	Coverage Δ
arc/species/converter.py	76.09% <0.00%> (ø)

... and 1 file with indirect coverage changes

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[kfir4444]

Let's wait for CI and merge.