Improve backup display names with emulated storage

-Change backup display name to say that we are excluding storage
-Add warning message during backup of any partition with data media
-Also eliminate unused variable from twrpTar class

Maybe this will make the people in issue 276 happy, but probably not because
they already lost their data, or because the warning text is yellow instead of
red.
https://github.com/TeamWin/Team-Win-Recovery-Project/issues/276

Change-Id: I98303fe7f6b7a25fea029637c90145258d41ee46