ssconf: error out when writing oversized files

Since we impose a maximum limit when reading ssconf files, let's error
out when trying to write them too big, so we don't pretend everything is
ok, and make mistakes when we actually read partial files.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>