Delete output apphost upon failure of the CreateAppHost task.

Port https://github.com/dotnet/sdk/pull/3427 by @peterhuene with my edit

This commit deletes the output apphost when the CreateAppHost task fails from
an exception.

Partially fixes dotnet/core-setup#2989

-------------
* deletes the output apphost when the `CreateAppHost` task fails from
an exception.

* aggreates the Exception for future logging from deleting the failed apphost


Commit migrated from https://github.com/dotnet/core-setup/commit/7f0eb2869f2f37a899cdd02b7bbc4bd78e4e7038

src/installer/managed/Microsoft.NET.HostModel/AppHost/FailedToDeleteApphostException.cs

+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+
+namespace Microsoft.NET.HostModel.AppHost
+{
+    /// <summary>
+    /// Failed to delete apphost when trying to delete incomplete appphost
+    /// </summary>
+    public class FailedToDeleteApphostException : AppHostUpdateException
+    {
+        public string ExceptionMessage { get; }
+        public FailedToDeleteApphostException(string exceptionMessage)
+        {
+            ExceptionMessage = exceptionMessage;
+        }
+    }
+}
+

src/installer/managed/Microsoft.NET.HostModel/AppHost/HostWriter.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
 using System;
+using System.Diagnostics;
 using System.IO;
 using System.IO.MemoryMappedFiles;
 using System.Text;
@@ -43,6 +44,8 @@ public static class HostWriter
 
             BinaryUtils.CopyFile(appHostSourceFilePath, appHostDestinationFilePath);
 
+            try
+            {
                 // Re-write the destination apphost with the proper contents.
                 bool appHostIsPEImage = false;
                 using (var memoryMappedFile = MemoryMappedFile.CreateFromFile(appHostDestinationFilePath))
@@ -83,6 +86,27 @@ public static class HostWriter
                 // Memory-mapped write does not updating last write time
                 File.SetLastWriteTimeUtc(appHostDestinationFilePath, DateTime.UtcNow);
             }
+            catch (Exception ex)
+            {
+                FailedToDeleteApphostException failedToDeleteApphostException = null;
+                // Delete the destination file so we don't leave an unmodified apphost
+                try
+                {
+                    File.Delete(appHostDestinationFilePath);
+                }
+                catch (Exception failedToDeleteEx) when (failedToDeleteEx is IOException || failedToDeleteEx is UnauthorizedAccessException)
+                {
+                    failedToDeleteApphostException = new FailedToDeleteApphostException(failedToDeleteEx.Message);
+                }
+
+                if (failedToDeleteApphostException != null)
+                {
+                    throw new AggregateException(ex, failedToDeleteApphostException);
+                }
+
+                throw;
+            }
+        }
 
         /// <summary>
         /// Set the current AppHost as a single-file bundle.