Report errors processing messages from the build host (#76252)

Related to #75967

Previously, any errors processing messages would just cause an inifnite
hang. This is updates the implementation to do two things
1.  Catch the specific string that failed to be deserialized (if any)
2. If processing failed for any reason, report the exception to the
outstanding requests and trigger disconnect.

src/Workspaces/Core/MSBuild/Rpc/RpcClient.cs

@@ -46,46 +46,63 @@ public void Start()
         // We'll start this and let it run until Shutdown has been called.
         Task.Run(async () =>
         {
-            string? line;
-            while ((line = await _receivingStream.TryReadLineOrReturnNullIfCancelledAsync(_shutdownTokenSource.Token).ConfigureAwait(false)) != null)
+            Exception? processingException = null;
+            try
             {
-                var response = JsonConvert.DeserializeObject<Response>(line);
+                string? line;
+                while ((line = await _receivingStream.TryReadLineOrReturnNullIfCancelledAsync(_shutdownTokenSource.Token).ConfigureAwait(false)) != null)
+                {
+                    Response? response;
+                    try
+                    {
+                        response = JsonConvert.DeserializeObject<Response>(line);
+                    }
+                    catch (JsonException ex)
+                    {
+                        var message = $"Failed to deserialize response from build host:{Environment.NewLine}{line}";
+                        throw new AggregateException(message, ex);
+                    }
 
-                Contract.ThrowIfNull(response);
+                    Contract.ThrowIfNull(response);
 
-                Contract.ThrowIfFalse(_outstandingRequests.TryRemove(response.Id, out var completionSourceAndExpectedType), $"We got a response for request ID {response.Id} but that was already completed.");
-                var (completionSource, expectedType) = completionSourceAndExpectedType;
+                    Contract.ThrowIfFalse(_outstandingRequests.TryRemove(response.Id, out var completionSourceAndExpectedType), $"We got a response for request ID {response.Id} but that was already completed.");
+                    var (completionSource, expectedType) = completionSourceAndExpectedType;
 
-                if (response.Exception != null)
-                {
-                    completionSource.SetException(new RemoteInvocationException(response.Exception));
-                }
-                else
-                {
-                    // If this is void-returning, then just set null
-                    if (expectedType == null)
+                    if (response.Exception != null)
                     {
-                        completionSource.SetResult(null);
+                        completionSource.SetException(new RemoteInvocationException(response.Exception));
                     }
                     else
                     {
-                        try
+                        // If this is void-returning, then just set null
+                        if (expectedType == null)
                         {
-                            // response.Value might be null if the response was in fact null.
-                            var result = response.Value?.ToObject(expectedType);
-                            completionSource.SetResult(result);
+                            completionSource.SetResult(null);
                         }
-                        catch (Exception ex)
+                        else
                         {
-                            completionSource.SetException(new Exception("Unable to deserialize the result", ex));
+                            try
+                            {
+                                // response.Value might be null if the response was in fact null.
+                                var result = response.Value?.ToObject(expectedType);
+                                completionSource.SetResult(result);
+                            }
+                            catch (Exception ex)
+                            {
+                                completionSource.SetException(new Exception("Unable to deserialize the result", ex));
+                            }
                         }
                     }
                 }
             }
+            catch (Exception ex)
+            {
+                processingException = ex;
+            }
 
             // We've disconnected, so cancel any remaining outstanding tasks
             foreach (var (request, _) in _outstandingRequests.Values)
-                request.SetException(new System.Exception("The server disconnected unexpectedly."));
+                request.SetException(processingException ?? new System.Exception("The server disconnected unexpectedly."));
 
             Disconnected?.Invoke(this, EventArgs.Empty);
         });