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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

:exclamation: 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

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