Re-implement correctly the handling of failure to open the database

If we can’t open it at startup, we exit.
If we can’t open it on reload, we keep the previously-opened database.

This way, we’re assured to always have a valid and open database available.

src/utils/reload.cpp

@@ -11,7 +11,6 @@ void open_database()
 #ifdef USE_DATABASE
   const auto db_filename = Config::get("db_name", xdg_data_path("biboumi.sqlite"));
   log_info("Opening database: ", db_filename);
-  Database::close();
   Database::open(db_filename);
   log_info("database successfully opened.");
 #endif