Returns the currently logged in user, or undef if there is none.
Normally the user is re-retrieved from the store.
-For L<Catalyst::Authentication::Store::DBIx::Class> the user is re-restored
-using the primary key of the user table.
+For L<Catalyst::Authentication::Store::DBIx::Class> the user is re-restored
+using the primary key of the user table.
Thus B<user> can throw an error even though B<user_exists>
returned true.