而不是 retrieve 的 WebClient ，你可以用 exchange 它允许您处理错误并使用从服务响应检索的消息传播自定义异常。

private void execute()
{
    WebClient webClient = WebClient.create();

    webClient.get()
             .uri("http://localhost:8089")
             .exchangeToMono(this::handleResponse)
             .doOnNext(System.out::println)
             .block();  // not required, just for testing purposes
}

private Mono<Response> handleResponse(ClientResponse clientResponse)
{
    if (clientResponse.statusCode().isError())
    {
        return clientResponse.bodyToMono(Response.class)
                             .flatMap(response -> Mono.error(new RuntimeException(response.message)));
    }

    return clientResponse.bodyToMono(Response.class);
}

private static class Response
{
    private String message;

    public Response()
    {
    }

    public String getMessage()
    {
        return message;
    }

    public void setMessage(String message)
    {
        this.message = message;
    }

    @Override
    public String toString()
    {
        return "Response{" +
                "message='" + message + '\'' +
                '}';
    }
}

下面的代码现在可以工作了，这是一个不同于我最初的问题的代码，但它的想法基本相同（使用后端restapi和数据库restapi）。
我的数据库rest api:

@RestController
@RequestMapping("/user")
public class UserControl {

    @Autowired
    UserRepo userRepo;

    @Autowired
    UserMapper userMapper;

    @GetMapping("/{login}")
    public Mono<UserDTO> getUser(@PathVariable String login) throws DatabaseException {
        User user = userRepo.findByLogin(login);
        if(user == null) {
            throw new DatabaseException(HttpStatus.BAD_REQUEST, "error.user.not.found");
        }
        return Mono.just(userMapper.toDTO(user));
    }
}

userrepo只是一个@restrepritory。
usermapper使用mapstruct将实体Map到dto对象。
使用：

@Data
@EqualsAndHashCode(callSuper=false)
public class DatabaseException extends ResponseStatusException {

    private static final long serialVersionUID = 1L;

    public DatabaseException(String message) {
        super(HttpStatus.BAD_REQUEST, message);
    }
}

@data&equalsandhashcode来自lombok库。
扩展responsestatusexception在这里非常重要，如果不这样做，那么响应将被错误处理。
从数据库rest api接收数据的后端rest api：

@RestController
@RequestMapping("/user")
public class UserControl {

    @Value("${database.api.url}")
    public String databaseApiUrl;

    private String prefixedURI = "/user";

    @GetMapping("/{login}")
    public Mono<UserDTO> getUser(@PathVariable String login) {
        return WebClient
                .create(databaseApiUrl)
                .get()
                .uri(prefixedURI + "/{login}", login).retrieve()
                .onStatus(HttpStatus::isError, GlobalErrorHandler::manageError)
                .bodyToMono(UserDTO.class);
    }
}

使用globalerrorhandler:：

public class GlobalErrorHandler {

    /**
     * Translation key for i18n
     */
    public final static String I18N_KEY_ERROR_TECHNICAL_EXCEPTION = "error.technical.exception";

    public static Mono<ResponseStatusException> manageError(ClientResponse clientResponse) {

        if (clientResponse.statusCode().is4xxClientError()) {
            // re-throw original status and message or they will be lost
            return clientResponse.bodyToMono(ExceptionResponseDTO.class).flatMap(response -> {
                return Mono.error(new ResponseStatusException(response.getStatus(), response.getMessage()));
            });
        } else { // Case when it's 5xx ClientError
            // User doesn't have to know which technical exception has happened
            return clientResponse.bodyToMono(ExceptionResponseDTO.class).flatMap(response -> {
                return Mono.error(new ResponseStatusException(HttpStatus.INTERNAL_SERVER_ERROR,
                        I18N_KEY_ERROR_TECHNICAL_EXCEPTION));
            });
        }

    }
}

以及ExceptionResponseTo，它是从clientresponse检索某些数据所必需的：

/**
 * Used to map <a href="https://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/web/reactive/function/client/ClientResponse.html">ClientResponse</a> from WebFlux 
 */
@Data
@EqualsAndHashCode(callSuper=false)
public class ExceptionResponseDTO extends Exception {

    private static final long serialVersionUID = 1L;

    private HttpStatus status;

    public ExceptionResponseDTO(String message) {
        super(message);
    }

    /**
     * Status has to be converted into {@link HttpStatus}
     */
    public void setStatus(String status) {
        this.status = HttpStatus.valueOf(Integer.valueOf(status));
    }

}

另一个可能有用的相关类：exchangefilterfunctions.java
我在这个问题上发现了很多信息：
https://github.com/spring-projects/spring-framework/issues/20280
即使这些信息是旧的，它们仍然是相关的！