ringdht: check IceTransport::start() returns

IceTransport::start() return was not checked
causing error not handled correctly, resulting
by an unstable application.

This patch adds the necessary checks and request
the associated call to finish by a failure state
when such error happens.

Change-Id: I3f07f0b4d0cf0bbc0f7e3704c4cd0139b4c5e64e
Tuleap: #662