Handle error returned by GetClusterInfos() (#30919)

A recent change to the enterprise cluster code introduced a change to the enterprise API interface. GetClusterInfos() can now return an error. This commit introduces code to handle that error.

server/channels/testlib/cluster.go

@@ -35,7 +35,7 @@ func (c *FakeClusterInterface) IsLeader() bool { return false }
 
 func (c *FakeClusterInterface) GetMyClusterInfo() *model.ClusterInfo { return nil }
 
-func (c *FakeClusterInterface) GetClusterInfos() []*model.ClusterInfo { return nil }
+func (c *FakeClusterInterface) GetClusterInfos() ([]*model.ClusterInfo, error) { return nil, nil }
 
 func (c *FakeClusterInterface) SendClusterMessage(message *model.ClusterMessage) {
 	c.mut.Lock()