Fix for Issue #1125 - Bootstrapper throws with multiple RootPathProviders

Actually, this doesn't FIX the bootstrapper, rather makes the error very specific so that when it occurs the reason is obvious.

The MultipleRootPathProvidersLocatedException captures the list of IRootPathProvider types located in a custom ProviderTypes property, and as a Data element named "ProviderTypes", so that information is readily available, and can be inspected.

What I wasn't able to do was figure out a sensible way to test this change, as the GetRootPathProvider() in the NancyBootstrapperBase class is private, static, and uses the static AppDomainAssemblyTypeScanner class directly. Some (minor) reworking needed to allow the list of located types to be faked would be needed.