Clarify exception message for repeated builders.

Technically these methods fail either if the field is primitive (non-message) or if the field is repeated.  However it is confusing to have a repeated field that claims to be of type 'message' that then fails with an error message that claims you need a message type.

PiperOrigin-RevId: 689042972

java/core/src/main/java/com/google/protobuf/GeneratedMessage.java

@@ -2709,13 +2709,12 @@ public void clear(final Builder<?> builder) {
 
       @Override
       public Message.Builder newBuilder() {
-        throw new UnsupportedOperationException(
-            "newBuilderForField() called on a non-Message type.");
+        throw new UnsupportedOperationException("newBuilderForField() called on a repeated field.");
       }
 
       @Override
       public Message.Builder getBuilder(GeneratedMessage.Builder<?> builder) {
-        throw new UnsupportedOperationException("getFieldBuilder() called on a non-Message type.");
+        throw new UnsupportedOperationException("getFieldBuilder() called on a repeated field.");
       }
 
       @Override