Error on custom force

[mattwthompson]

See #248

[mikemhenry]

This doesn't fix the issue entirely but provides better error messages

[codecov-commenter]

Codecov Report

Patch coverage: 60.00% and project coverage change: +0.02 🎉

Comparison is base (8660a1a) 69.21% compared to head (0af09b9) 69.23%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #292      +/-   ##
==========================================
+ Coverage   69.21%   69.23%   +0.02%     
==========================================
  Files           5        5              
  Lines         851      855       +4     
==========================================
+ Hits          589      592       +3     
- Misses        262      263       +1     

Impacted Files	Coverage Δ
...penmmforcefields/generators/template_generators.py	74.91% <60.00%> (+<0.01%)	⬆️

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[mattwthompson]

Yup - "short-term band-aid" for some values of "short-term" 🤣

I wont' be able to dig deeply enough into this to fix it within the next couple of days, but I figure it's better to stop the bleeding and put better boundaries on what behavior is tested and somewhat trustworthy. Up to you either way!