Include details when umount fails
Include details when umount fails

Add the mountpoint and the rc value to the FSError message. Details and
output of lsof are still available in program.log, this just makes the
error more clear to whoever catches it.

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