feat(tour) show list of topics when user tries to view topic that doesn't exist

eg.

```
ipfs2 (cmd-ref-part2) λ. go build ./...; ./ipfs2 tour 0

Tour 0 - Hello Mars

Hello Mars

ipfs2 (cmd-ref-part2) λ. go build ./...; ./ipfs2 tour 10

ERROR
    no topic with id: 10

TOPICS

    0 - Hello Mars
    0.1 - Hello Mars 2

```

cmd/ipfs2/tour.go

@@ -11,8 +11,12 @@ import (
 	config "github.com/jbenet/go-ipfs/config"
 	internal "github.com/jbenet/go-ipfs/core/commands2/internal"
 	tour "github.com/jbenet/go-ipfs/tour"
+	"github.com/jbenet/go-ipfs/util"
 )
 
+// TODO the parent function now uses tourOutput. Migrate the children to also
+// use the tourOutput struct
+
 var tourCmd = &cmds.Command{
 	Helptext: cmds.HelpText{
 		Tagline: "An introduction to IPFS",
@@ -33,38 +37,101 @@ IPFS very quickly. To start, run:
 		"next":    cmdIpfsTourNext,
 		"restart": cmdIpfsTourRestart,
 	},
-	Run: func(req cmds.Request) (interface{}, error) {
+	Run: tourRunFunc,
+	Marshalers: cmds.MarshalerMap{
+		cmds.Text: tourTextMarshaler,
+	},
+	Type: &tourOutput{},
+}
 
-		out := new(bytes.Buffer)
-		cfg, err := req.Context().GetConfig()
-		if err != nil {
-			return nil, err
-		}
+// tourOutput is a union type. It either contains a Topic or it contains the
+// list of Topics and an Error.
+type tourOutput struct {
+	Topic *tour.Topic
 
-		strs, err := internal.CastToStrings(req.Arguments())
-		if err != nil {
-			return nil, err
-		}
+	Topics []tour.Topic
+	Error  error
+}
 
-		id := tour.TopicID(cfg.Tour.Last)
-		if len(strs) > 0 {
-			id = tour.TopicID(strs[0])
-		}
+func tourTextMarshaler(r cmds.Response) ([]byte, error) {
+	output, ok := r.Output().(*tourOutput)
+	if !ok {
+		return nil, util.ErrCast()
+	}
+	// can be listing when error
+	var buf bytes.Buffer
+	err := printTourOutput(&buf, output)
+	if err != nil {
+		return nil, err
+	}
+	return buf.Bytes(), nil
+}
 
-		t, err := tourGet(id)
-		if err != nil {
-			return nil, cmds.ClientError(err.Error())
-		}
+func printTourOutput(w io.Writer, output *tourOutput) error {
+	tmpl := `{{ if .Error }}
+ERROR
+	{{ .Error }}
+TOPICS
+	{{ range $topic := .Topics }}
+	{{ $topic.ID }} - {{ $topic.Title }} {{ end }}
+{{ else if .Topic }}
+Tour {{ .Topic.ID }} - {{ .Topic.Title }}
 
-		err = tourShow(out, t)
-		if err != nil {
-			return nil, err
+{{ .Topic.Text }}
+{{ end }}
+`
+	tourTmpl, err := template.New("tour").Parse(tmpl)
+	if err != nil {
+		return err
+	}
+	return tourTmpl.Execute(w, output)
+}
+
+func tourRunFunc(req cmds.Request) (interface{}, error) {
+
+	cfg, err := req.Context().GetConfig()
+	if err != nil {
+		return nil, err
+	}
+
+	strs, err := internal.CastToStrings(req.Arguments())
+	if err != nil {
+		return nil, err
+	}
+
+	id := tour.TopicID(cfg.Tour.Last)
+	if len(strs) > 0 {
+		id = tour.TopicID(strs[0])
+	}
+
+	t, err := tourGet(id)
+	if err != nil {
+
+		// If no topic exists for this id, we handle this error right here.
+		// To help the user achieve the task, we construct a response
+		// comprised of...
+		// 1) a simple error message
+		// 2) the full list of topics
+
+		output := &tourOutput{
+			Error: err,
+		}
+		for _, id := range tour.IDs {
+			t, ok := tour.Topics[id]
+			if !ok {
+				return nil, err
+			}
+			output.Topics = append(output.Topics, t)
 		}
 
-		return out, nil
-	},
+		return output, nil
+		// return nil, cmds.ClientError(err.Error())
+	}
+
+	return &tourOutput{Topic: t}, nil
 }
 
+// TODO use tourOutput like parent command
 var cmdIpfsTourNext = &cmds.Command{
 	Helptext: cmds.HelpText{
 		Tagline: "Show the next IPFS Tour topic",
@@ -97,7 +164,7 @@ var cmdIpfsTourNext = &cmds.Command{
 		}
 
 		w.WriteTo(os.Stdout) // TODO write to res.SetValue
-		return nil, nil
+		return w, nil
 	},
 }
 
@@ -122,6 +189,7 @@ var cmdIpfsTourRestart = &cmds.Command{
 	},
 }
 
+// TODO use tourOutput like parent command
 var cmdIpfsTourList = &cmds.Command{
 	Helptext: cmds.HelpText{
 		Tagline: "Show a list of IPFS Tour topics",
@@ -171,6 +239,7 @@ Tour {{ .ID }} - {{ .Title }}
 	return ttempl.Execute(w, t)
 }
 
+// tourGet returns an error if topic does not exist
 func tourGet(id tour.ID) (*tour.Topic, error) {
 	t, found := tour.Topics[id]
 	if !found {

cmd/ipfs2/tour_test.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"bytes"
+	"errors"
 	"testing"
 
 	"github.com/jbenet/go-ipfs/tour"
@@ -11,8 +12,9 @@ func TestParseTourTemplate(t *testing.T) {
 	topic := &tour.Topic{
 		ID:    "42",
 		Title: "IPFS CLI test files",
-		Text: `Welcome to the IPFS test files
-		This is where we test our beautiful command line interfaces
+		Text: `
+Welcome to the IPFS test files
+This is where we test our beautiful command line interfaces
 		`,
 	}
 	var buf bytes.Buffer
@@ -22,3 +24,48 @@ func TestParseTourTemplate(t *testing.T) {
 	}
 	t.Log(buf.String())
 }
+
+func TestRenderTourOutputList(t *testing.T) {
+
+	t.Log(`Ensure we can successfully print the tour output when there's an
+	error and list of tour topics`)
+	listOutput := &tourOutput{
+		Error: errors.New("Topic 42 does not exist"),
+		Topics: []tour.Topic{
+			tour.Topic{
+				ID:    "41",
+				Title: "Being one shy of the mark",
+				Text:  "Poor thing.",
+			},
+			tour.Topic{
+				ID:    "44",
+				Title: "Two shy of the mark",
+				Text:  "Oh no.",
+			},
+		},
+	}
+
+	var list bytes.Buffer
+	if err := printTourOutput(&list, listOutput); err != nil {
+		t.Fatal(err)
+	}
+	t.Log(list.String())
+}
+
+func TestRenderTourOutputSingle(t *testing.T) {
+	t.Log(`
+	When there's just a single topic in the output, ensure we can render the
+	template`)
+	singleOutput := &tourOutput{
+		Topic: &tour.Topic{
+			ID:    "42",
+			Title: "Informative!",
+			Text:  "Compelling!",
+		},
+	}
+	var single bytes.Buffer
+	if err := printTourOutput(&single, singleOutput); err != nil {
+		t.Fatal(err)
+	}
+	t.Log(single.String())
+}