LanguageClient: Do not send out invalid errors

An error is expected to contain an error message so add one that misses
one. Additionally assert when trying to set an error to a response that
is invalid (does not contain message or code.)

Fixes: QTCREATORBUG-28598
Fixes: QTCREATORBUG-27856
Change-Id: I0631433d847610531758d297a93a5c7cdfdc6c1d
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/libs/languageserverprotocol/jsonrpcmessages.h

@@ -290,7 +290,10 @@ public:
         return val.isUndefined() ? std::nullopt : std::make_optional(fromJsonValue<Error>(val));
     }
     void setError(const Error &error)
-    { m_jsonObject.insert(errorKey, QJsonValue(error)); }
+    {
+        QTC_CHECK(error.isValid());
+        m_jsonObject.insert(errorKey, QJsonValue(error));
+    }
     void clearError() { m_jsonObject.remove(errorKey); }
 
     bool isValid(QString *errorMessage) const override

src/plugins/languageclient/client.cpp

@@ -1891,6 +1891,7 @@ void ClientPrivate::handleMethod(const QString &method, const MessageId &id, con
         Response<JsonObject, JsonObject> response(id);
         ResponseError<JsonObject> error;
         error.setCode(ResponseError<JsonObject>::MethodNotFound);
+        error.setMessage(QString("The client cannot handle the method '%1'.").arg(method));
         response.setError(error);
         sendResponse(response);
     }