Clarify error in applying forcefield

[jaclark5]

PR Summary:

Slight change in error string to provide guidance to a user on how to resolve it.

PR Checklist

---

• [NA] Includes appropriate unit test(s)
• [NA] Appropriate docstring(s) are added/updated
• [NA] Code is (approximately) PEP8 compliant
• [NA] Issue(s) raised/addressed?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.55%. Comparing base (cbbc4c8) to head (39c137f).
Report is 13 commits behind head on main.

❗ Current head 39c137f differs from pull request most recent head 10a521d. Consider uploading reports for the commit 10a521d to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #549   +/-   ##
=======================================
  Coverage   69.55%   69.55%           
=======================================
  Files          16       16           
  Lines        1662     1662           
=======================================
  Hits         1156     1156           
  Misses        506      506