Improve handling of configuration file
    
     * Allow MBS_CONFIG_FILE="" to entirely suppress loading any configuration
       file (useful for running tests and avoiding loading a system-wide
       configuration file.)
     * When loading the configuration file:
       * If the default configuration file path doesn't exist, silently fall back
         to the default configuration
       * For any other OSError, print the exact error
       * Let any other exception throw through, to allow people to debug their
         configuration file