Wrong error message for when `DidChangeTextDocument` path doesn't exist

[alidn]

https://github.com/rust-analyzer/rust-analyzer/blob/796bfccac4215fc753f805cc61bb87a64a06b5f6/crates/rust-analyzer/src/main_loop.rs#L663

Here the error message should either be 'unexpected DidChangeTextDocument' or 'expected DidOpenTextDocument'