Print error if run dir is not a directory

Print an error message if the run directory or the device-class
subdirectory exists but is not a directory.

Before:

    % rm -rf /tmp/brightnessctl
    % touch /tmp/brightnessctl
    % brightnessctl -s
    Device 'intel_backlight' of class 'backlight':
            Current brightness: 1000 (20%)
            Max brightness: 4794

    % file /tmp/brightnessctl
    /tmp/brightnessctl: empty
    %

After:

    % brightnessctl -s
    Error saving device data: Not a directory
    Could not save data for device 'intel_backlight'.
    Device 'intel_backlight' of class 'backlight':
            Current brightness: 1000 (20%)
            Max brightness: 4794

    %

0 files changed, 0 insertions, 0 deletions