Reworded registration exception for clarity

Ported dotnet/aspnetcore#12636

Author: alefranz

src/HeaderPropagationMessageHandler.cs

@@ -48,8 +48,8 @@ protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage reques
             {
                 var message =
                     $"The {nameof(HeaderPropagationValues)}.{nameof(HeaderPropagationValues.Headers)} property has not been " +
-                    $"initialized. Register the header propagation middleware by adding 'app.{nameof(HeaderPropagationApplicationBuilderExtensions.UseHeaderPropagation)}() " +
-                    $"in the 'Configure(...)' method.";
+                    $"initialized. Register the header propagation middleware by adding 'app.{nameof(HeaderPropagationApplicationBuilderExtensions.UseHeaderPropagation)}()' " +
+                    $"in the 'Configure(...)' method. Header propagation can only be used within the context of an HTTP request.";
                 throw new InvalidOperationException(message);
             }