Gift Exchange: "Edit Prompt" on Offer does not provide errors if the resulting offer is invalid

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.