Fix two issues with exports and snapshot errors

This patch fixes two issues related to failed snapshots during exports:
  - first, the error messages used disk.logical_id[1], which is a node
    name for DRBD, and it resulted in strange error messages like
    "cannot snapshot block device node1 on node2"
  - second, if snapshotting fails for any disk, rpc.call_finalize_export
    fails as it didn't handle booleans (backend.FinalizeExport does)

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