Move throw into interface to use in wizard, check for null and invalid username.

MediaBrowser.Controller/Library/IUserManager.cs

@@ -33,6 +33,12 @@ namespace MediaBrowser.Controller.Library
         /// <value>The users ids.</value>
         IEnumerable<Guid> UsersIds { get; }
 
+        /// <summary>
+        /// Checks if the user's username is valid.
+        /// </summary>
+        /// <param name="name">The user's username.</param>
+        void ThrowIfInvalidUsername(string name);
+
         /// <summary>
         /// Initializes the user manager and ensures that a user exists.
         /// </summary>