log/sequencer: no new root is not an error (#1041)
log/sequencer: no new root is not an error (#1041)

Repeat of commit b053d2e8d611a564ab which got lost in a
sequence of merges.
gdbelvin/trillian,gdbelvin/trillian,AlCutter/trillian,Martin2112/trillian,Martin2112/trillian,Martin2112/trillian,google/trillian,RJPercival/trillian,daviddrysdale/trillian,daviddrysdale/trillian,RJPercival/trillian,gdbelvin/trillian,google/trillian,daviddrysdale/trillian,AlCutter/trillian