Exceptions in Yesod
Philosophically speaking, in the Haskell/Yesod world you want to pass the values forward, rather than return them backwards. So instead of having the handlers return a value, have them call forwards to the next step in the process, which may be to generate an exception.
Remember that you can bundle any amount of future actions into a single object, so you can pass a continuation object to your handlers and foos that basically tells them, "After you are done, run this blob of code." That way they can be void and return nothing.