roachtest: better failure message for c2c roachtests

In some failures cases the c2c roachtests may try to cutover even though the
replication did not start successfully. This patch makes sure we fail with
a more clear error in those cases.

Epic: none
Fixes: #107935

Release note: None

pkg/cmd/roachtest/tests/cluster_to_cluster.go

@@ -749,6 +749,8 @@ func (rd *replicationDriver) main(ctx context.Context) {
 	rd.checkParticipatingNodes(ingestionJobID)
 
 	retainedTime := rd.getReplicationRetainedTime()
+	require.GreaterOrEqual(rd.t, cutoverTime, retainedTime,
+		"cannot cutover to a time below the retained time (did the test already fail?)")
 
 	rd.metrics.cutoverTo = newMetricSnapshot(metricSnapper, cutoverTime)
 	rd.metrics.cutoverStart = newMetricSnapshot(metricSnapper, timeutil.Now())