releasetools: Fix error text in ota_from_target_files

property isn't the variable name, prop is

Change-Id: I0fc6adddfd51ba9bc79f6559c61cd831203c9e3d
Merge remote-tracking branch 'cyanogen/cm-11.0' into cm-11.0
Merge remote-tracking branch 'cm/cm-11.0' into cm-11.0