Allow custom RPC method errors

[MOZGIII]

I'd like to do something like this:

#[rpc(server, client, namespace = "myrpc")]
pub trait MyRpc {
    // an explicit error for this method
    #[method(name = "myMethod1")]
    async fn my_method1(&self) -> Result<MyData1, MyError1>;

    // an explicit error for this method
    #[method(name = "myMethod2")]
    async fn my_method2(&self) -> Result<MyData2, MyError2>;

    // backward-compatible generic error for this method
    #[method(name = "myMethod3")]
    async fn my_method3(&self) -> Result<MyData3, jsonrpsee::core::Error>;
}

pub enum MyError1 {
  // some variants here
}

pub enum MyError2 {
  // some variants here
}

impl From<MyError1> for jsonrpsee::core::Error {
    fn from(val: MyError1) -> jsonrpsee::core::Error {
        // my custom implementation for this method's errors
        jsonrpsee::core::Error::Other(ErrorObjectOwned { /*  ... */ }}
    }
}

impl From<MyError2> for jsonrpsee::core::Error {
    fn from(val: MyError2) -> jsonrpsee::core::Error {
        // my custom implementation for this method's errors
        jsonrpsee::core::Error::Other(ErrorObjectOwned { /*  ... */ }}
    }
}

The idea here is to allow methods to return an error type that can be converted into jsonrpsee::core::Error instead of requiring it explicitly.

This would allow us to have type-safe and clearly expressed application-level errors for the RPC calls - something we have had trouble with for quite some time.

We want to supply the data for certain errors, and our current implementation is a mess; people tend to manually construct the jsonrpsee::core::Error instead of going through the helper functions (which are not really discoverable, at least not in a way the type-safe errors enable).

Our frontend guys are also complaining it's hard to understand all possible application-level error cases as the code evolves, and we figured having an explicit error enum for every method would work very well for us.

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This can be implemented in a backward-compatible way if the error is made to be impl Into<jsonrpsee::core::Error> instead of an explicit jsonrpsee::core::Error.

Alternatively, Into<jsonrpsee::types:error::ErrorObjectOwned> could be used - this is more direct, but doesn't allow invoking the baked-in errors, which is kind of a limitation.

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This does not make much sense from the client-side perspective, so I propose the client trait to rewrite the errors to still be jsonrpsee::core::Error.