Since this stack pieces will never error, we can mark their
`Error`s with a type that can "never" be created. When seeing an `Error
= ()`, it can either mean the error never happens, or that the detailed
error is dealt with elsewhere and only a unit is passed on. When seeing
`Error = Never`, it is clearer that the error case never happens.
Besides helping humans, LLVM can also remove the error branchs entirely.
Signed-off-by: Sean McArthur <sean@buoyant.io>
Sean McArthur