Add a new extrinsic `dispatch_as_fallible`.

It's almost the same as [`Pallet::dispatch_as`] but forwards any error
of the inner call.

Closes #219.

And add more unit tests to cover `dispatch_as` and
`dispatch_as_fallible`.

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Signed-off-by: Xavier Lau <x@acg.box>
Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Bastian Köcher <info@kchr.de>
Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>