Make error message in setDefaultFSType more informative (#1019766)
Make error message in setDefaultFSType more informative (#1019766)

Related: rhbz#1019766

This has been observed to raise a ValueError a couple of times. Want to
make the error message more informative in case it happens again.

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

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