save-cmd.py: insist on having an index; check for the primary index file.

save-cmd.py: insist on having an index; check for the primary index file.

Insist on having an index, and report an error (with a suggestion to
run "bup index") if one isn't found.

Previously (at least as recently as 0.25-rc1), bup save would just
create an empty tree if there was no index.  The metadata work
unintentionally broke that behavior.

For now, treat a missing index as an error under the assumption that
it's not likely to be intentional.  If a good use case for the
previous behavior is presented, we can always restore it.

Signed-off-by: Jean-Baptiste Denis <jbd@jbdenis.net>
[rlb@defaultvalue.org: adjust commit message and save-cmd.py error message.]
Signed-off-by: Rob Browning <rlb@defaultvalue.org>