ReshardingTest fixture must accommodate nodes going into ROLLBACK while attempting to run replSetStepUp

A node closes its user connections as part of its transition to member state ROLLBACK. The ReshardingTest#stepUpNewPrimaryOnShard() method should swallow any network error when attempting to run the replSetStepUp command and move on to attempting to step up another secondary node.

const res = newPrimary.adminCommand({replSetStepUp: 1});

if (res.ok === 1) {

    replSet.awaitNodesAgreeOnPrimary();

    assert.eq(newPrimary, replSet.getPrimary());

    return;

}