If reload content failed, don’t update it

In case user wants a fresh version of the current one and the website isn’t available, don’t erase it with a boring message saying wallabag wasn’t able to refresh the content.

src/Wallabag/CoreBundle/DependencyInjection/Configuration.php

@@ -39,6 +39,8 @@ class Configuration implements ConfigurationInterface
                 ->integerNode('cache_lifetime')
                     ->defaultValue(10)
                 ->end()
+                ->scalarNode('fetching_error_message')
+                ->end()
             ->end()
         ;