Improve error message for IoT node without a tunnel

When a node doesn't have an active tunnel, the error should say so
clearly. This is reflected in 2 places:
- proxy error log
- tsh user output

Also, use trace.UserMessage instead of utils.UserMessageFromError.
utils.UserMessageFromError will return a full stack trace when calling
process (proxy) is in debug mode. The user shouldn't see this.

lib/reversetunnel/api.go

@@ -18,6 +18,7 @@ package reversetunnel
 
 import (
 	"context"
+	"fmt"
 	"net"
 	"time"
 
@@ -54,6 +55,14 @@ type DialParams struct {
 	ServerID string
 }
 
+func (params DialParams) String() string {
+	to := params.To.String()
+	if to == "" {
+		to = params.ServerID
+	}
+	return fmt.Sprintf("from: %q to: %q", params.From, to)
+}
+
 // RemoteSite represents remote teleport site that can be accessed via
 // teleport tunnel or directly by proxy
 //

lib/reversetunnel/localsite.go

@@ -181,7 +181,7 @@ func (s *localSite) Dial(params DialParams) (net.Conn, error) {
 }
 
 func (s *localSite) DialTCP(params DialParams) (net.Conn, error) {
-	s.log.Debugf("Dialing from %v to %v.", params.From, params.To)
+	s.log.Debugf("Dialing %v.", params)
 
 	conn, _, err := s.getConn(params)
 	if err != nil {
@@ -266,6 +266,11 @@ func (s *localSite) getConn(params DialParams) (conn net.Conn, useTunnel bool, e
 			return nil, false, trace.Wrap(err)
 		}
 
+		// This node can only be reached over a tunnel, don't attempt to dial
+		// remotely.
+		if params.To.String() == "" {
+			return nil, false, trace.ConnectionProblem(err, "node is offline, please try again later")
+		}
 		// If no tunnel connection was found, dial to the target host.
 		dialer := proxy.DialerFromEnvironment(params.To.String())
 		conn, err = dialer.DialTimeout(params.To.Network(), params.To.String(), defaults.DefaultDialTimeout)

lib/srv/regular/sshserver.go

@@ -1411,7 +1411,7 @@ func (s *Server) handleProxyJump(ccx *sshutils.ConnectionContext, identityContex
 
 func (s *Server) replyError(ch ssh.Channel, req *ssh.Request, err error) {
 	log.Error(err)
-	message := []byte(utils.UserMessageFromError(err))
+	message := []byte(trace.UserMessage(err))
 	ch.Stderr().Write(message)
 	if req.WantReply {
 		req.Reply(false, message)