prevent panic when receiving malformed LSP PublishDiagnostic (#2160)

Instead of panicing we can discard the malformed diagnostic. This
`.parse()` fails commonly when a non-conformant language server gives
a diagnostic with a location that breaks the spec:

    { "character": 0, "line": -1 }

can currently be returned by ElixirLS and the python LS. Other
messages in this block are discarded but this one feels special enough
to log.

helix-lsp/src/lib.rs

@@ -276,7 +276,13 @@ impl Notification {
             lsp::notification::PublishDiagnostics::METHOD => {
                 let params: lsp::PublishDiagnosticsParams = params
                     .parse()
-                    .expect("Failed to parse PublishDiagnostics params");
+                    .map_err(|err| {
+                        log::error!(
+                            "received malformed PublishDiagnostic from Language Server: {}",
+                            err
+                        )
+                    })
+                    .ok()?;
 
                 // TODO: need to loop over diagnostics and distinguish them by URI
                 Self::PublishDiagnostics(params)