GH-3276: reactive inbound: Fix onErrorResume

Fixes #3276

The `onErrorResume` for the `MessagingGatewaySupport.doSendAndReceiveMessageReactive()`
was in wrong place: only for the `buildReplyMono` which works only
when an outbound flow is fully based on reactive channels.
With a regular direct channel we can get an exception from the
`sendMessageForReactiveFlow` which is not covered with the mentioned
`onErrorResume` for the error handling on the configured `errorChannel`

Cherry-pick to `5.2.x & 5.1.x`

# Conflicts:
#	spring-integration-core/src/main/java/org/springframework/integration/gateway/MessagingGatewaySupport.java

Author: artembilan

spring-integration-core/src/main/java/org/springframework/integration/gateway/MessagingGatewaySupport.java

@@ -620,7 +620,8 @@ private Mono<Message<?>> doSendAndReceiveMessageReactive(MessageChannel requestC
 			sendMessageForReactiveFlow(requestChannel, requestMessage);
 
 			return buildReplyMono(requestMessage, replyChan, error, originalReplyChannelHeader,
-					originalErrorChannelHeader);
+					originalErrorChannelHeader)
+					.onErrorResume(t -> error ? Mono.error(t) : handleSendError(requestMessage, t));
 		});
 	}
 
@@ -670,8 +671,7 @@ private Mono<Message<?>> buildReplyMono(Message<?> requestMessage, FutureReplyCh
 								.setHeader(MessageHeaders.ERROR_CHANNEL, originalErrorChannelHeader)
 								.build();
 					}
-				})
-				.onErrorResume(t -> error ? Mono.error(t) : handleSendError(requestMessage, t));
+				});
 	}
 
 	private Mono<Message<?>> handleSendError(Message<?> requestMessage, Throwable exception) {