Toast error message not displayed on failure to delete custom property

Overview:
On attempting to delete a custom property that is in use (i.e. there is a value set for the property on at least one agreement/license) the user should see an error message in a toast in the UI.

Steps to Reproduce:

1. Create a supplementary property in Agreement settings
2. Create an agreement and set a value for the supplementary property
3. Attempt to delete the supplementary property in Agreement settings

Expected Results:
Property should fail to delete and user should see toast message indicating the error. Previously this read:
“There was an error deleting the supplementary property. org.hibernate.exception.ConstraintViolationException: could not execute statement”

Actual Results:
Property fails to delete but no feedback for user