Unable to unmount disk
Probably something went wrong wiping the disk. You can't properly erase/repartition a disk the OS X booted from. Additionally it seems that you don't have a proper Recovery HD.

I recommend either to create a bootable installer thumb drive with the Lion installer or booting to Internet Recovery Mode by pressing altcmdR while booting, formatting the internal disk and restoring OS X. Not all Macs boot to IRM though.

To create a bootable thumb drive you need the Lion installer app, a second Mac and a USB thumb drive (8 GB or more)