Return HTTP 500 when failing to load the JSON configuration

[nodiscc]

Currently when the syntax of config.json.php is invalid, Shaarli returns a HTTP 200 (successful), but displays the following message.

An error occurred while parsing JSON configuration file (data/config.json.php): error code #4
➜ Syntax error
Please check your JSON syntax (without PHP comment tags) using a JSON lint tool such as jsonlint.com.

It should reply with a HTTP code 500 instead. This would make it easier to monitor the actual status of a Shaarli instance.