Make vars_del_variable() set errno when the variable size is bad.

Make vars_del_variable() set errno when the variable size is bad.

Commit eaaedefb correctly changes this code path so that it tests the
file size correctly, but errno is still untouched, so from the
consumer's point of view, if the real error *does* occur, you'll get
something like:

Could not delete variable: Success

which is obviously nonsense.  Instead, errno should be something about
the size.  So I picked EFBIG.

Signed-off-by: Peter Jones <pjones@redhat.com>