Use available types from ReflectionTypeLoadException when finding Hubs

- provide better tracing when Assembly.GetTypes() throws

#2712
Decided it was better not to use types from ReflectionTypeLoadException

In practice, these types are often not usable by the HubMethodDispatcher,
e.g. a FileNotFound exception can be thrown when compiling an Expression
Added a trace listener for initialization errors/warnings to the sample

- So far this only listens to the SignalR.ReflectedHubDescriptorProvider key