[Feat] Provide warnings in roll dialogs when incorrect configuration is provided

[Pjb518]

We should test whether a roll formula is valid in the roll dialog and prevent form submission if submitting will result in an error.

• [ ] Add a helper function to test a roll formula
• [ ] Prevent roll dialog form submission when the roll formula is invalid
• [ ] Provide a warning within the dialog indicating that the roll formula is invalid

This could be improved further by highlighting the specific roll terms which are problematic.