GH-3418: Warn on ack failure in the AbstractAdaptableMessageListener

Fixes: #3418

The message could be ack'ed in the error handler without any evidence and clue
for the `AbstractAdaptableMessageListener` to determine what to do with afterward.

* Use `logger.warn()` instead of `error` in the `AbstractAdaptableMessageListener.basicAck()`
when `channel.basicAck()` fails for the provided message, which, essentially, might be already
acke'ed this or other way in the user's error handler

(cherry picked from commit b7f773a)

Author: artembilan

spring-rabbit/src/main/java/org/springframework/amqp/rabbit/listener/adapter/AbstractAdaptableMessageListener.java

@@ -453,7 +453,7 @@ protected void basicAck(Message request, @Nullable Channel channel) {
 				channel.basicAck(request.getMessageProperties().getDeliveryTag(), false);
 			}
 			catch (IOException e) {
-				this.logger.error("Failed to ack message", e);
+				this.logger.warn("Failed to ack message", e);
 			}
 		}
 	}