fix: add compiler error for using Result with init

[agostbiro]

As reported in #926, currently an #[init] method that returns a Result, but misses a #[handle_result] annotation compiles successfully, but causes problems at runtime.

This PR fixes that, so writing

#[init]
pub fn new() -> Result<Self, &'static str> { }

Now fails to compile with the

Serializing Result<T, E> has been deprecated. Consider marking your method with #[handle_result]...

message, but writing

#[init]
#[handle_result]
pub fn new() -> Result<Self, &'static str> { }

compiles successfully as before.

[frol]

@agostbiro Thanks for the contribution! This looks good and has the necessary test checking for it, so I am ready to merge it once CI is green.