contract delegator bug

Created by: NetGodFather

Edit code base on https://github.com/paritytech/ink/tree/master/examples/delegator Modify adder/lib.rs，Add code like this:

#[ink(message)]
pub fn x_inc(&mut self, by: i32) -> Result<(), Error>{
    self.accumulator.inc(by);
    Ok(())
}

Compiling will report a large number of errors。

Add the following code again:

#[derive(Debug, PartialEq, Eq, scale::Encode)]
#[cfg_attr(feature = "std", derive(scale_info::TypeInfo))]
pub enum Error {
}

Compiling will return the following error:

Compiling adder v3.0.0-rc3 (/private/var/folders/fx/y3j80rgn433961wkpcjq4_fr0000gn/T/cargo-contract_sUXlX7/adder)
error[E0599]: the method `fire` exists for struct `CallBuilder<DefaultEnvironment, Set<ink_env::AccountId>, Unset<u64>, Unset<u128>, Set<ExecutionInput<ArgumentList<ink_env::call::utils::Argument<i32>, ArgumentList<ArgumentListEnd, ArgumentListEnd>>>>, Set<ReturnType<Result<(), adder::Error>>>>`, but its trait bounds were not satisfied
   --> /Users/hezhiqun/Substrate/ink/examples/delegator/adder/lib.rs:54:9
    |
54  | /         pub fn x_inc(&mut self, by: i32) -> Result<(), Error>{
55  | |             self.accumulator.inc(by);
56  | |
57  | |             Ok(())
58  | |         }
    | |_________^ method cannot be called on `CallBuilder<DefaultEnvironment, Set<ink_env::AccountId>, Unset<u64>, Unset<u128>, Set<ExecutionInput<ArgumentList<ink_env::call::utils::Argument<i32>, ArgumentList<ArgumentListEnd, ArgumentListEnd>>>>, Set<ReturnType<Result<(), adder::Error>>>>` due to unsatisfied trait bounds
    |
   ::: /Users/hezhiqun/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/result.rs:241:1
    |
241 |   pub enum Result<T, E> {
    |   --------------------- doesn't satisfy `_: adder::_::_parity_scale_codec::Decode`
    |
    = note: the following trait bounds were not satisfied:
            `Result<(), adder::Error>: adder::_::_parity_scale_codec::Decode`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0599`.
error: could not compile `adder`

To learn more, run the command again with --verbose.