Add .uninhabited_error() next to .unit_error()

[PvdBerg1998]

I recently rewrote my code to use futures 0.3. One of the issues I ran into was that hyper requires a future 0.1 where the error type implements std::error::Error. The natural solution to this would be to use .unit_error(), but this does not implement the required trait.

For my own purposes I basically copy-pasted the implementation of UnitError, but with the error type set to !, which implements every trait.

I would suggest deprecating .unit_error() in favour of .uninhabited_error() since it's more representative of what is actually happening (the Future::Output will always be Result::Ok), but since for example tokio currently requires spawned futures to have a unit error this would be unergonomic.