Return ContractReverted from dispatchables when top level contract reverted

[athei]

Contracts have the possibility to set a flag that reverts all the changes they made. The calling contract can evaluate these flags and act on it. However, if the top level contract sets the revert flag when returning the extrinsic is counted as Ok. We should introduce a new pallet level error Err(ContractReverted) that is returned instead.

Please note that seal_return allows to return a result buffer that can be used to encode additional error information. The buffer is only accessible when calling via RPC of course. How this buffer is to be decoded (by a UI) for a specific contract should be defined by the userspace ABI (json). When called via RPC we return Ok so that buffer is accessible. UIs should inspect the flags field in order to determine whether the call was a success.

Please note that this should only happen if the call/instantiate was submitted as transaction where the flags field cannot be read. An RPC call should still return OK and it is up to the UI to classify it as error by reading flags. Reason is that on dispatch error no output buffer could be returned.