Improve delegate! error handling

[Krzysztof Pobiarżyn]

If you make any mistake in a delegate block, errors are swallowed.

The block expects a trait function to be declared, so without the pub keyword.
Any other syntax errors cause that all the delegated functions don't show up in the module.
The macro should emit syn errors to be presented in IDE.

delegate! {
        to self.erc20 {
            pub fn transfer(&mut self, recipient: &Address, amount: &U256); // pub
            fn transfer_from(&mut self, owner: &Address, recipient: &Address, amount: &U256);
            fn approve(&mut self, spender: &Address, amount: &U256);
            fn name(&self) -> String;
            fn symbol(&self) -> String;
            fn decimals(&self) - a-> u8; // wrong arrow
            fn total_supply(&self) -> U256;
            fn balance_of(&self, owner: &Address) -> U256;
            fn allowance(&self, owner: &Address, spender: &Address) -> U256;
        }

        to self.ownable {
            fn get_owner(&self) -> Address;
            fn transfer_ownership(&mut self, new_owner: &Address);
        }
    }