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.


Note: the following PR will be based on this branch.