Gift Exchange: "Edit Prompt" on Offer does not provide errors if the resulting offer is invalid
Description
Steps to reproduce
Create a sign-up for an open gift exchange
Edit your offers via the "Edit Prompt" button on the "My Signup" page in such a way that it no longer meets the challenge's criteria
Select the "Update" button
What happens
Nothing. The page reloads in its edit state, with the changes made reflected, but no error message is made and the edit isn't applied
What should happen
If you do the same thing on the Requests version of "Edit Prompt" you get a red flash error message describing what element no longer meets its criteria. This should also happen on the Offers.
Activity
Can confirm that on both the Request and Offers pages when I try to change the element from an accepted element to an unaccepted element, I get a red flash error message.
Also received the red flash error message describing the element that no longer met criteria when I attempted to edit both Requests and Offers.
