swaybg: check for exact number of arguments
swaybg: check for exact number of arguments

christophgysin/sway,1ace/sway,sleep-walker/sway,colemickens/sway,christophgysin/sway,crondog/sway,colemickens/sway,taiyu-len/sway,sleep-walker/sway,taiyu-len/sway,mikkeloscar/sway,ptMuta/sway,colemickens/sway,ascent12/sway,taiyu-len/sway,mikkeloscar/sway,1ace/sway,4e554c4c/sway,ascent12/sway,crondog/sway,ascent12/sway,1ace/sway,4e554c4c/sway,SirCmpwn/sway,johalun/sway