Added error message to error response

src/main/java/com/nisum/ascend/promotions/controller/PromotionsController.java

@@ -32,7 +32,7 @@ public class PromotionsController {
     public Mono<PromotionDto> getPromotionById(@PathVariable String id){
         return promotionService
                 .findPromoById(id)
-                .onErrorMap(throwable -> new PromotionNotFoundException(id));
+                .switchIfEmpty(Mono.error(new PromotionNotFoundException(id)));
     }
 
     @PostMapping()

src/main/java/com/nisum/ascend/promotions/exception/PromotionNotFoundException.java

 package com.nisum.ascend.promotions.exception;
 
+import org.springframework.http.HttpStatus;
+import org.springframework.web.bind.annotation.ResponseStatus;
+
+@ResponseStatus(value = HttpStatus.NOT_FOUND)
 public class PromotionNotFoundException extends RuntimeException {
     public PromotionNotFoundException(String id) {
         super("The promotion with the ID of " + id + " was not found.");

src/main/java/com/nisum/ascend/promotions/exception/RestWebExceptionHandler.java

 package com.nisum.ascend.promotions.exception;
 
 import org.springframework.core.annotation.Order;
+import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.http.HttpStatus;
 import org.springframework.stereotype.Component;
 import org.springframework.web.server.ServerWebExchange;
 import org.springframework.web.server.WebExceptionHandler;
+import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
+import springfox.documentation.spring.web.json.Json;
+
+import java.nio.charset.StandardCharsets;
 
 @Component
 @Order(-2)
@@ -15,9 +20,9 @@ class RestWebExceptionHandler implements WebExceptionHandler {
     public Mono<Void> handle(ServerWebExchange exchange, Throwable ex) {
         if (ex instanceof PromotionNotFoundException) {
             exchange.getResponse().setStatusCode(HttpStatus.NOT_FOUND);
-
+            DataBuffer buffer = exchange.getResponse().bufferFactory().wrap(ex.getMessage().getBytes(StandardCharsets.UTF_8));
             // marks the response as complete and forbids writing to it
-            return exchange.getResponse().setComplete();
+            return exchange.getResponse().writeWith(Flux.just(buffer));
         }
         return Mono.error(ex);
     }