Log clearer error on startup if NPC cannot be started

Servers should always be upgraded before agents, but generally this
isn't required because things are compatible between versions. In this
case we're OK with failing closed if the user upgrades out of order, but
we should give a clearer message about what steps are required to fix
the issue.
Signed-off-by: Brad Davidson <brad.davidson@rancher.com>

pkg/agent/config/config.go

@@ -498,6 +498,13 @@ func get(envInfo *cmds.Agent, proxy proxy.Proxy) (*config.Node, error) {
 		nodeConfig.AgentConfig.ServiceNodePortRange = *controlConfig.ServiceNodePortRange
 	}
 
+	// Old versions of the server do not send enough information to correctly start the NPC. Users
+	// need to upgrade the server to at least the same version as the agent, or disable the NPC
+	// cluster-wide.
+	if controlConfig.DisableNPC == false && (controlConfig.ServiceIPRange == nil || controlConfig.ServiceNodePortRange == nil) {
+		return nil, fmt.Errorf("incompatible down-level server detected; servers must be upgraded to at least %s, or restarted with --disable-network-policy", version.Version)
+	}
+
 	nodeConfig.AgentConfig.ExtraKubeletArgs = envInfo.ExtraKubeletArgs
 	nodeConfig.AgentConfig.ExtraKubeProxyArgs = envInfo.ExtraKubeProxyArgs