8397: Return proper error code when server is loading r=matklad a=ceronman
When requests are made to rust-analyzer and the server is still loading, a response error is returned with the code `ContentModified` and text `"Rust Analyzer is still loading..."`. This error code doesn't seem to be the more appropriate for this situation. Using `ServerNotInitialized` seems better.
As this is such a small change, I have not created an issue for it.
Co-authored-by: Manuel Ceron <manuel.ceron@jetbrains.com>