Check that file that loop device is going to use exists (#982164)
Check that file that loop device is going to use exists (#982164)

Related: rhbz#982164

Catches the non-existance of the file slightly earlier and gives a more
informative message instead of just failing when losetup is called.

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

rhinstaller/blivet,vojtechtrefny/blivet,vojtechtrefny/blivet,AdamWill/blivet,rvykydal/blivet,rhinstaller/blivet,jkonecny12/blivet,vpodzime/blivet,dwlehman/blivet,AdamWill/blivet,rvykydal/blivet,vpodzime/blivet,dwlehman/blivet,jkonecny12/blivet