Expanding message registration exception message as it is a little misleading

The exception message states that the messages must implement, IMessage, IEvent or ICommand however this may not have been the cause of the exception. If the user has configured NSB such that the message types have never been registered (e.g. NSB has not scanned the required assembly) then they will also get this error message even though their messages implement the correct interfaces.
Updated exception message in MessageMetadataRegistry to give extra guidance when message types cannot be found.
Added newlines to improve readability and updated file scanning URL to point to particular.net
Changed \n to Environment.NewLine in MessageMetadata exception message to provide better cross-platform support
Merge pull request #1598 from fergusontom/develop

Expanding message registration exception message as it is a little misleading