Finalize local export only if successful

Finalize local export only if successful

Before commit 44247302, a failure in the transfer of a single disk
during an export was changed to make the entire transfer unsuccessful.
What did not change was an invocation of export finalization, which
removes the old backup if present and replaces it with whatever the
result of the transfer was. As a result, a failed backup of an instance
could destroy a previous backup.

This patch prevents the finalization from taking place - this leaves
some data lying around in a ".new" directory, but at least the old
backup is not deleted.

Signed-off-by: Hrvoje Ribicic <riba@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>