Return a better error message for when we are waiting for cargo.

crates/rust-analyzer/src/main_loop.rs

@@ -484,7 +484,7 @@ fn on_request(&mut self, request_received: Instant, req: Request) -> Result<()>
                 req.id,
                 // FIXME: i32 should impl From<ErrorCode> (from() guarantees lossless conversion)
                 lsp_server::ErrorCode::ContentModified as i32,
-                "Rust Analyzer is still loading...".to_owned(),
+                "waiting for cargo metadata or cargo check".to_owned(),
             ));
             return Ok(());
         }