frontend expects the backend to return errors, but the backend lies and says everything was successful, and then certain things break in the UI

That’s a double failure then: not only does the backend do it wrong, the frontend devs don’t even know it. If they’d agreed on one way of handling it, they’d still be able to work it out. But if the devs don’t even communicate their standards with each other and the frontend devs obviously don’t know about the problem…