Throw AuthenticationException instead of ArgumentNullException when a user does not exist

Emby.Server.Implementations/Library/DefaultAuthenticationProvider.cs

@@ -47,7 +47,7 @@ namespace Emby.Server.Implementations.Library
         {
             if (resolvedUser == null)
             {
-                throw new ArgumentNullException(nameof(resolvedUser));
+                throw new AuthenticationException($"Specified user does not exist.");
             }
 
             bool success = false;