Return an LspResult from LSP Request handlers

[DavisVaughan]

I'm starting to look at posit-dev/positron#8350, and I believe that statement_range() is going to need to be able to return a structured jsonrpc::Error with a well defined code and message that we can pick up on on the frontend.

In particular, I'm likely going to have it return RequestFailed
https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#responseMessage

/**
	 * A request failed but it was syntactically correct, e.g the
	 * method name was known and the parameters were valid. The error
	 * message should contain human readable information about why
	 * the request failed.
	 *
	 * @since 3.17.0
	 */
	export const RequestFailed: [integer](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#integer) = -32803;

Currently all of our LSP Request handlers return an anyhow::Result, and at the very last second we convert any anyhow::Errors into a jsonrpc::Error with an arbitrary -1 code. This is no longer going to be good enough for cases where we need to return structured jsonrpc::Errors with a specific code.

Luckily I think we can get the best of both worlds, here's what I've come up with for the return type of all Request handlers:

pub(crate) type LspResult<T> = std::result::Result<T, LspError>;

#[derive(Debug)]
pub(crate) enum LspError {
    JsonRpc(jsonrpc::Error),
    Anyhow(anyhow::Error),
}

This comes with nice From methods

// For the ability to `?` a `jsonrpc::Error` into an `LspError`
impl From<jsonrpc::Error> for LspError {
    fn from(error: jsonrpc::Error) -> Self {
        Self::JsonRpc(error)
    }
}

// For the ability to `?` an `anyhow::Error` into an `LspError`
impl From<anyhow::Error> for LspError {
    fn from(error: anyhow::Error) -> Self {
        Self::Anyhow(error)
    }
}

which means there is very little code to change. We can still use ? on any helper method that returns an anyhow::Result, and the anyhow::Error will get wrapped into LspError::Anyhow automatically.

I haven't actually done anything in statement_range() yet, but I'm planning on returning something like this using this new LspResult feature:

// Unfortunately `tower_lsp::jsonrpc::ErrorCode::RequestFailed` isn't provided yet.
const REQUEST_FAILED: i64 = -32803;
return LspResult::Err(LspError::JsonRpc(jsonrpc::Error {
    code: jsonrpc::ErrorCode::ServerError(REQUEST_FAILED),
    message: Cow::Owned(String::from("Parse failure")),
    data: None,
}));

[DavisVaughan]

I recognized along the way that Crashed is only ever intended to contain an anyhow::Error, i.e. it will never hold the Ok() variant of an anyhow::Result<>, so we can simplify this a little.

[DavisVaughan]

This is most of the magic. We can now build a context aware jsonrpc::Error at the failure site and pipe it all the way through to be sent to the frontend. I think this will be useful for more than just statement_range() going forward.

[DavisVaughan]

A little bit of fiddling to get this right, but I think it reads a little clearer in the end