Report error when instantiating a contract from a non-payable message

[cmichi]

I'm creating this issue as a follow-up for #740.

The issue is that if a contract has a method a la

#[ink(message)]
pub fn deploy(&mut self, version: u32, code_hash: Hash) {
    let _ = Subcontract::new(b)
        .endowment(self.env().balance() / 4)
        .code_hash(code_hash)
        .salt_bytes(version.to_le_bytes())
        .instantiate()
        .expect("failed at instantiating);
}

Then calling this method will fail at runtime with ContractTrapped. This is because the payable attribute is missing, the message signature should be #[ink(message, payable)].

In an ideal world we would detect this already at compile time, though I'm not sure how feasible this is with nested functions and what not.

Either way: We should improve the error reporting and documentation on this in some way.

[cmichi]

Closing the issue, since the payable is not actually necessary, just verified it. The contract pays entirely from it's own balance for the deployment.