Do not close an attached window because the owner answered badly

src/DiffEngineViewer.Tests/AttachedViewerTests.cs

@@ -151,6 +151,33 @@ public async Task ABusyOwnerDoesNotReadAsDead()
         _ = listening;
     }
 
+    /// <summary>
+    /// An owner that answers is an owner, whatever it answered.
+    /// <para>
+    /// ViewerServer turns any exception in the listing handler into an error reply, and this side
+    /// read one of those as the owner having gone: the window closed with "The queue owner is no
+    /// longer running." over a single transient throw, taking the queue it was displaying with it.
+    /// </para>
+    /// </summary>
+    [Test]
+    public async Task AnErrorReplyIsNotADeadOwner()
+    {
+        await Assert.That(ViewerServer.TryBind(0, out var bound)).IsTrue();
+        using var server = bound!;
+        using var cancel = new CancelSource();
+        var listening = server.Listen(
+            _ => ViewerResponse.Error("The listing could not be built"),
+            cancel.Token);
+        var host = new SessionHost(SessionState.Start(ViewerMode.Inline, Fixtures.Columns, Fixtures.Rows));
+
+        await Assert.That(new OwnerLink(host, server.Port).Pump()).IsTrue();
+
+        await Assert.That(host.State.Message).IsEqualTo("The listing could not be built");
+        await Assert.That(host.State.Exit).IsFalse();
+        await cancel.CancelAsync();
+        _ = listening;
+    }
+
     /// <summary>
     /// The owner has no window of its own, so raising, hiding and closing come back on a listing
     /// rather than being pushed at a port this process does not hold.

src/DiffEngineViewer/Ipc/OwnerLink.cs

@@ -61,12 +61,24 @@ public bool Pump(out bool sent)
             message = Send(command);
         }
 
-        if (!ViewerClient.TrySend(new(ViewerVerb.ListFull), out var response, port, Wait) ||
-            !response.Ok)
+        if (!ViewerClient.TrySend(new(ViewerVerb.ListFull), out var response, port, Wait))
         {
             return false;
         }
 
+        if (!response.Ok)
+        {
+            // An owner that answers is an owner. ViewerServer turns any exception in the listing
+            // handler into an error reply, so reading one as death closed this window over a
+            // single transient throw and lost the queue it was displaying. Said instead, and asked
+            // again on the next pass.
+            host.Mutate(_ => _ with
+            {
+                Message = response.Message ?? "The queue owner refused the listing."
+            });
+            return true;
+        }
+
         var pending = InlineQueue.From(ViewerListing.Pending(response.Items));
         var changes = ReadChanges(response);
         host.Mutate(_ => ViewerSession.Sync(_, pending, changes, message));