Fix error checking while changing wallpaper.

[stephentoth]

In response to #138
Fixes being able to assign a wallpaper to a nonexistent monitor by throwing an error if the monitor doesn't exist.
Clarified README to the specifics of the handleWallpaper function.

Without really modifying the existing logic for the if (MONITOR.empty()) { part there isn't a neat way of just assigning a wallpaper to all screens at once.

This does fix the issue aswell as follows:

$ hyprctl hyprpaper listactive
eDP-1 = <wp1>
HDMI-A-1 = <wp2>

 $ hyprctl hyprpaper preload <wp3>
ok

$ hyprctl hyprpaper wallpaper ", <wp3>"
ok
$ hyprctl hyprpaper listactive
eDP-1 = <wp1>
HDMI-A-1 = <wp2>

$ hyprctl hyprpaper wallpaper "ramd, <wp3>"
wallpaper failed (no such monitor)
$ hyprctl hyprpaper listactive
eDP-1 = <wp1>
HDMI-A-1 = <wp2>

[vaxerski]

oke