[ADD] CDPErrorTag in http response header

CometServer/Modules/10-25/ApiBase.cs

@@ -1047,5 +1047,24 @@ protected CometTask CreateAndRegisterCometTask(Guid actor, string topContainerNa
 
             return cometTask;
         }
+
+        /// <summary>
+        /// Adds a specific CDP error tag to an <see cref="HttpResponse"/> message
+        /// </summary>
+        /// <param name="httpResponse">The <see cref="HttpResponse"/></param>
+        /// <param name="errorTag">The the Error tag</param>
+        protected void AddErrorTagHeader(HttpResponse httpResponse, string errorTag)
+        {
+            var errorTagHeader = "CDPErrorTag";
+
+            if (!httpResponse.Headers.ContainsKey(errorTagHeader))
+            {
+                httpResponse.Headers[errorTagHeader] = errorTag;
+            }
+            else
+            {
+                httpResponse.Headers.TryAdd(errorTagHeader, errorTag);
+            }
+        }
     }
 }

CometServer/Modules/10-25/EngineeringModelApi.cs

@@ -1027,7 +1027,9 @@ protected async Task PostResponseData(PostRequestData postRequestData, string re
 
                     if (!IsPostAllowedOnIterationSetup(resourceProcessor, iteration))
                     {
-                        throw new Cdp4ModelValidationException($"It is not allowed to write data to a Frozen {nameof(IterationSetup)}, or its full containment tree. (#FROZEN_ITERATION).");
+                        this.AddErrorTagHeader(httpResponse, "#FROZEN_ITERATION");
+
+                        throw new Cdp4ModelValidationException($"It is not allowed to write data to a Frozen {nameof(IterationSetup)}, or its full containment tree.");
                     }
 
                     credentialsService.Credentials.Iteration = iteration;