Add missing provider_unauthroized method to import_github.rb

lib/api/import_github.rb

@@ -20,6 +20,10 @@ module API
       def provider
         :github
       end
+      
+      def provider_unauthorized
+        error!("Access denied to your #{Gitlab::ImportSources.title(provider.to_s)} account.", 500)
+      end
     end
 
     desc 'Import a GitHub project' do