revise operation result encoding and add more comments.

src/DurableTask.Core/Entities/EventFormat/ResponseMessage.cs

@@ -30,13 +30,13 @@ internal class ResponseMessage : EntityMessage
         public FailureDetails? FailureDetails { get; set; }
 
         [IgnoreDataMember]
-        public bool IsErrorResult => this.ErrorMessage != null;
+        public bool IsErrorResult => this.ErrorMessage != null || this.FailureDetails != null;
 
         public override string GetShortDescription()
         {
             if (this.IsErrorResult)
             {
-                return $"[OperationFailed {this.ErrorMessage} {this.FailureDetails}]";
+                return $"[OperationFailed {this.FailureDetails?.ErrorMessage ?? this.ErrorMessage}]";
             }
             else if (this.Result == LockAcquisitionCompletion)
             {

src/DurableTask.Core/Entities/OperationFormat/OperationResult.cs

@@ -23,19 +23,27 @@ public class OperationResult
 
         /// <summary>
         /// The serialized result returned by the operation. Can be null, if the operation returned no result.
-        /// May contain error details, such as a serialized exception, if <see cref="ErrorMessage"/> is not null.
+        /// May contain error details, such as a serialized exception, if <see cref="IsError"/> is true.
         /// </summary>
         public string? Result { get; set; }
 
+        /// <summary>
+        /// Whether this operation completed successfully.
+        /// </summary>
+        public bool IsError
+            => this.ErrorMessage != null || this.FailureDetails != null;
+
         /// <summary>
         /// If non-null, this string indicates that this operation did not successfully complete. 
-        /// The actual content and its interpretation varies depending on the SDK used. 
+        /// The content and interpretation varies depending on the SDK used. For newer SDKs,
+        /// we rely on the <see cref="FailureDetails"/> instead.
         /// </summary>
         public string? ErrorMessage { get; set; }
 
         /// <summary>
         /// A structured language-independent representation of the error. Whether this field is present
-        /// depends on which SDK is used, and on configuration settings.
+        /// depends on which SDK is used, and on configuration settings. For newer SDKs, we use
+        /// this field exclusively when collecting error information.
         /// </summary>
         public FailureDetails? FailureDetails { get; set; }
     }

src/DurableTask.Core/Logging/LogEvents.cs

@@ -1228,7 +1228,7 @@ public EntityBatchExecuted(EntityBatchRequest request, EntityBatchResult result)
                 this.InstanceId = request.InstanceId;
                 this.OperationCount = request.Operations.Count;
                 this.ResultCount = result.Results.Count;
-                this.ErrorCount = result.Results.Count(x => x.ErrorMessage != null);
+                this.ErrorCount = result.Results.Count(x => x.IsError);
                 this.ActionCount = result.Actions.Count;
                 this.EntityStateLength = request.EntityState?.Length ?? 0;
             }

src/DurableTask.Core/TaskEntityDispatcher.cs

@@ -250,12 +250,15 @@ protected async Task<bool> OnProcessWorkItemAsync(TaskOrchestrationWorkItem work
 
                         // if we encountered an error, record it as the result of the operations
                         // so that callers are notified that the operation did not succeed.
-                        if (result.FailureDetails != default(FailureDetails))
+                        if (result.FailureDetails != null)
                         {
                             OperationResult errorResult = new OperationResult()
                             {
-                                Result = null,
+                                // for older SDKs only
+                                Result = result.FailureDetails.ErrorMessage,
                                 ErrorMessage = "entity dispatch failed",
+
+                                // for newer SDKs only
                                 FailureDetails = result.FailureDetails,
                             };