Improve service connection error message with connection ID

src/Agent.Worker/TaskCommandExtension.cs

@@ -793,7 +793,7 @@ public void Execute(IExecutionContext context, Command command)
             var endpoint = context.Endpoints.Find(a => a.Id == endpointId);
             if (endpoint == null)
             {
-                throw new ArgumentNullException(StringUtil.Loc("InvalidEndpointId"));
+                throw new ArgumentNullException(StringUtil.Loc("ServiceConnectionNotDeclared", endpointId));
             }
 
             if (String.Equals(field, "url", StringComparison.OrdinalIgnoreCase))

src/Misc/layoutbin/en-US/strings.json

@@ -613,6 +613,7 @@
   "ServerUrl": "server URL",
   "ServiceAlreadyExists": "The service already exists: {0}, it will be replaced",
   "ServiceConfigured": "Service {0} successfully configured",
+  "ServiceConnectionNotDeclared": "Service connection with id '{0}' is not declared in the pipeline job and therefore can't be referenced in task.setendpoint commands.",
   "ServiceDelayedStartOptionSet": "Service {0} successfully set to delayed auto start",
   "ServiceInstalled": "Service {0} successfully installed",
   "ServiceLockErrorRetry": "Service DB lock failed with code {0}. Retrying after {1} seconds...",

src/Test/L0/Worker/TaskCommandExtensionL0.cs

@@ -150,6 +150,25 @@ public void SetEndpointInvalidEndpointId()
             }
         }
 
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void SetEndpointServiceConnectionNotDeclared()
+        {
+            using (var _hc = SetupMocks())
+            {
+                TaskCommandExtension commandExtension = new TaskCommandExtension();
+                var cmd = new Command("task", "setEndpoint");
+                cmd.Data = "http://example.com/";
+                cmd.Properties.Add("field", "url");
+                // Use a valid GUID that is not in the Endpoints list (SetupMocks only adds Guid.Empty)
+                var testEndpointId = "12345678-1234-1234-1234-123456789012";
+                cmd.Properties.Add("id", testEndpointId);
+
+                Assert.Throws<ArgumentNullException>(() => commandExtension.ProcessCommand(_ec.Object, cmd));
+            }
+        }
+
         [Fact]
         [Trait("Level", "L0")]
         [Trait("Category", "Worker")]