Ensure that the "Messages" portion of a PersistentResponse is an array

- .NET client
- Previously it was possible to iterate over an object instead
- Thanks to @JamesNK for pointing it out

https://github.com/SignalR/SignalR/commit/c20d822b46d60bdf2003e39c9c2e7b6a58873bf8#commitcomment-4729636

#2691
Skip bad assemblies when generating JS Hub proxies using signalr.exe

#2711
Report clearer error messages when duplicate Hubs are found

- This is particularly useful when trying to generate JS Hub proxies
  from signar.exe

#2687
#2711