pallet-scheduler: Send `CallUnavailable` event if the call isn't present (#1161)

The event was already send before, but it was done at the wrong
position. The pull request all changes the `execute_dispatch` signature
to highlight that there is only one error type.

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Javier Viola <javier@parity.io>