Providing details about the required free space

Adding a more meaningful text to better explain
how much free disk space you need to proceed

Change-Id: Iab06eeb467b6003a27bbe3566f0d64336f9d0728
Signed-off-by: Simone Tiraboschi <stirabos@redhat.com>
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1077150