button subsystem: use manual panic! instead of unwrap()

unwrap only works if the error value implements Debug, however since the
buttons subsystem doesn't return TockValue<E> but TockError this isn't the case
(anymore).  In theory one should think about refactoring check_user_presence()
to return TockResult<()> and possibly implementing some way for Ctap2StatusCode
to be converted to TockError enums or similar.  Then we can just pass errors
up the stack instead of panicking early.