Use official busybox binary, moved some "exit 1"

I was having problem's with CyanogenMod's dd, creating files with the wrong size. I decided to download the official busybox binary in order to avoid these inconsistencies: this script shouldn't be ROM-depended.

Also I added some exit 1 and removed some others: it happened that it couldn't download the archive for some reason and the script would go on and create the fs, mount it, try to extract the non-existing/corrupt archive on top of it then complain because it couldn't do stuff and exit leaving an unusable system.
I also removed the exit 1s on the bottom (after the umounts) because it may happen that something is not mounted, and that's not enough to skip all the following umounts.