Fix decoding of LangError

[ascjones]

As reported by @lean-apple, there was a bug decoding a LangError:

cargo contract call -m switch --dry-run --suri //Alice  --contract $CONTRACT
ERROR: Error decoding type 6: Result

Caused by:
    0: Error decoding type 8: ink_primitives::LangError
    1: No variant found with discriminant 1

This is because we were ignoring the explicitly assigned discriminant:

pub enum LangError {
    /// Failed to read execution input for the dispatchable.
    CouldNotReadInput = 1u32,
}

and instead using the implicit discriminant to decode.

The issue now is that the result is possibly not very useful to the user:

Perhaps we should consider explicitly decoding the LangError?