instance alternative exceptional

Stream: general

[Jannis Limperg]

I noticed that there's no instance of alternative for the exceptional from init.meta.exceptional. Should this instance be added? Two reasons I can think of not to do it:

• failure would rarely be useful since it doesn't give an informative error message (but then we could still have an instance has_orelse exceptional).
• orelse doesn't short-circuit like in Haskell, so can give unintuitive (if you come from Haskell) performance characteristics.