Separate method for unauthorized project messages

This commit separates out the logic for handling
unauthorized projects to reduce the complexity of the
authorized_project_find! method.

ee/lib/api/nuget_packages.rb

@@ -29,12 +29,16 @@ module API
         project = find_project(id)
 
         unless project && can?(current_user, :read_project, project)
-          return current_user ? not_found! : unauthorized_with_header!
+          return unauthorized_project_message
         end
 
         project
       end
 
+      def unauthorized_project_message
+        current_user ? not_found! : unauthorized_with_header!
+      end
+
       def unauthorized_with_header!
         header(AUTHENTICATE_REALM_HEADER, AUTHENTICATE_REALM_NAME)
         unauthorized!