fix: log error messages (#16935) (CP: 2.9) (#16963)

Log error messages and throw more
generic exceptions.

flow-server/src/main/java/com/vaadin/flow/server/communication/rpc/PublishedServerEventHandlerRpcHandler.java

@@ -27,6 +27,7 @@
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
+import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 import com.vaadin.flow.component.ClientCallable;
@@ -142,11 +143,12 @@ static void invokeMethod(Component instance, Class<?> clazz,
             invokeMethod(compositeContent, compositeContent.getClass(),
                     methodName, args, promiseId);
         } else {
-            String msg = String.format(
-                    "Neither class '%s' "
+            getLogger().error(String.format(
+                    "Faulty method invocation. Neither class '%s' "
                             + "nor its super classes declare event handler method '%s'",
-                    instance.getClass().getName(), methodName);
-            throw new IllegalStateException(msg);
+                    instance.getClass().getName(), methodName));
+            throw new IllegalStateException(
+                    "Faulty method invocation. See server log for more details.");
         }
     }
 
@@ -158,11 +160,12 @@ private static Optional<Method> findMethod(Component instance,
                         || method.isAnnotationPresent(ClientCallable.class))
                 .collect(Collectors.toList());
         if (methods.size() > 1) {
-            String msg = String.format(
-                    "Class '%s' contains "
-                            + "several event handler method with the same name '%s'",
-                    instance.getClass().getName(), methodName);
-            throw new IllegalStateException(msg);
+            getLogger().error(String.format(
+                    "Method conflict in event handler. Class '%s' contains "
+                            + "several event handler methods with the same name '%s'",
+                    instance.getClass().getName(), methodName));
+            throw new IllegalStateException(
+                    "Method conflict in event handler with multiple methods with same name. See server log for more details.");
         } else if (methods.size() == 1) {
             return Optional.of(methods.get(0));
         } else if (!Component.class.equals(clazz)) {
@@ -372,4 +375,9 @@ private static Collection<RpcDecoder> loadDecoders() {
         decoders.add(new DefaultRpcDecoder());
         return decoders;
     }
+
+    private static Logger getLogger() {
+        return LoggerFactory.getLogger(
+                PublishedServerEventHandlerRpcHandler.class.getName());
+    }
 }