Incorrect error message is displayed when the user deletes a supplementary property which is in use on an agreement

Overview:

When the user tries to delete a Supplementary Property which has been applied to an agreement the error message "There was an error deleting the supplementary property. org.hibernate.exception.ConstraintViolationException: could not execute statement" is displayed.

Steps to Reproduce:

1. Log into snapshot as diku_admin.
2. Open the Settings app and go to Agreements > Supplementary properties.
3. Add a new supplementary property.
4. In the Agreements app, Edit an agreement.
5. Apply the supplementary property created in step 3 and save the agreement.
6. Open the Settings app and go to Agreements > Supplementary properties.
7. "Delete" the supplementary property
8. Confirm the deletion (select the "Delete" button in the "Delete supplementary property" modal).

Expected Results:

A dismissable callout should be displayed with the text "Error: Supplementary property [property name] was not deleted because it is in use on one or more agreements." as shown in the mockup.

Actual Results:

A dismissible callout is displayed with the error message "There was an error deleting the supplementary property. org.hibernate.exception.ConstraintViolationException: could not execute statement"