LSP: consider RequestFailed for syntax errors

[amyreese]

Instead of sending or displaying warning/error messages, consider raising a RequestFailed error code:

raise JsonRpcException(
    code=-32803,
    message=...,
)

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.

https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#responseError