Refactor DataPipelineStatus and DataPipeline classes (#870)

Removed the `Failed` property from `DataPipelineStatus`.

## Motivation and Context (Why the change? What's the scenario?)
See #709

service/Abstractions/Models/DataPipelineStatus.cs

@@ -12,13 +12,8 @@ public class DataPipelineStatus
     [JsonPropertyName("completed")]
     public bool Completed { get; set; } = false;
 
-    [JsonPropertyOrder(1)]
-    [JsonPropertyName("failed")]
-    public bool Failed { get; set; } = false;
-
     /// <summary>
     /// Currently used to track deleted documents.
-    /// TODO: replace with "isDeleting" and "Deleted"
     /// </summary>
     [JsonPropertyOrder(2)]
     [JsonPropertyName("empty")]

service/Abstractions/Pipeline/DataPipeline.cs

@@ -154,11 +154,7 @@ public void MarkProcessedBy(IPipelineStepHandler handler, string? subStep = null
         /// <param name="text">Text to store for the end user</param>
         public void Log(IPipelineStepHandler handler, string text)
         {
-            if (this.LogEntries == null)
-            {
-                this.LogEntries = new List<PipelineLogEntry>();
-            }
-
+            this.LogEntries ??= [];
             this.LogEntries.Add(new PipelineLogEntry(source: handler.StepName, text: text));
         }
     }
@@ -438,7 +434,6 @@ public DataPipelineStatus ToDataPipelineStatus()
         return new DataPipelineStatus
         {
             Completed = this.Complete,
-            Failed = false, // TODO
             Empty = this.Files.Count == 0,
             Index = this.Index,
             DocumentId = this.DocumentId,