converted MPD_ERROR_ constants to enum
converted MPD_ERROR_ constants to enum

cmende/libmpdclient,jwise/libmpdclient,jagleeso/libmpdclient,yusiwen/libmpdclient,jwise/libmpdclient,yusiwen/libmpdclient,OpenInkpot-archive/iplinux-libmpdclient,yusiwen/libmpdclient,OpenInkpot-archive/iplinux-libmpdclient,cmende/libmpdclient,psych0tik/libmpdclient2,jagleeso/libmpdclient,jwise/libmpdclient,sl0th/libmpdclient,sl0th/libmpdclient,sl0th/libmpdclient,cmende/libmpdclient,psych0tik/libmpdclient2,jagleeso/libmpdclient