Add example which demonstrates difference in behavior of return type Result for contracts

[cmichi]

This is a follow-up to #665.

We should create an example to illustrate how revert-on-Err works. The example should especially highlight that Result behaves different if returned from a root contract vs. returned from child contracts.

ToDo

• For this purpose the example should have root and a child contract: The child contract should expose a message that returns a Result. The root contract should have a message returning Result as well.
• The comments should highlight that in the case of root returning Err a revert will happen, whereas for the child contract's Result return no revert will happen.
• As a follow-up: create an issue in ink-waterfall to test this new example.