Resolve unreachable_patterns warnings from nightly.

Nightly rust doesn't like it when you have a match arm that can never be reached because of an uninhabited type. As such, we can't say stuff like:

let x: Option<Void> = ...;
match x {
   Some(_) => unreachable!(),
   None => ...
}

Assigning review to @gabi-250 since part of this touches hsservice publisher code.