imlib.c: Give slightly more helpful error messages if exec fails
imlib.c: Give slightly more helpful error messages if exec fails

BlackLight/feh,Caellian/feh,Caellian/feh,BlackLight/feh,yusiwen/feh,yusiwen/feh,BlackLight/feh,Caellian/feh,yusiwen/feh,Caellian/feh