fix: error message for trust policy (#933)

This error message improvement is based on the Notation CLI error message improvement guideline that we defined in #834

Resolve part of #934 
Signed-off-by: Junjie Gao <junjiegao@microsoft.com>

cmd/notation/policy/import.go

@@ -42,7 +42,12 @@ func importCmd() *cobra.Command {
 Example - Import trust policy configuration from a file:
   notation policy import my_policy.json
 `,
-		Args: cobra.ExactArgs(1),
+		Args: func(cmd *cobra.Command, args []string) error {
+			if len(args) != 1 {
+				return fmt.Errorf("requires 1 argument but received %d.\nUsage: notation policy import <path-to-policy.json>\nPlease specify a trust policy file location as the argument", len(args))
+			}
+			return nil
+		},
 		RunE: func(cmd *cobra.Command, args []string) error {
 			opts.filePath = args[0]
 			return runImport(cmd, opts)
@@ -71,7 +76,7 @@ func runImport(command *cobra.Command, opts importOpts) error {
 	// optional confirmation
 	if !opts.force {
 		if _, err := trustpolicy.LoadDocument(); err == nil {
-			confirmed, err := cmdutil.AskForConfirmation(os.Stdin, "Existing trust policy configuration found, do you want to overwrite it?", opts.force)
+			confirmed, err := cmdutil.AskForConfirmation(os.Stdin, "The trust policy file already exists, do you want to overwrite it?", opts.force)
 			if err != nil {
 				return err
 			}

cmd/notation/policy/show.go

@@ -15,7 +15,9 @@ package policy
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
+	"io/fs"
 	"os"
 
 	"github.com/notaryproject/notation-go/dir"
@@ -53,13 +55,16 @@ func runShow(command *cobra.Command, opts showOpts) error {
 	// get policy file path
 	policyPath, err := dir.ConfigFS().SysPath(dir.PathTrustPolicy)
 	if err != nil {
-		return fmt.Errorf("failed to obtain path of trust policy configuration file: %w", err)
+		return fmt.Errorf("failed to obtain path of trust policy file: %w", err)
 	}
 
 	// core process
 	policyJSON, err := os.ReadFile(policyPath)
 	if err != nil {
-		return fmt.Errorf("failed to load trust policy configuration, you may import one via `notation policy import <path-to-policy.json>`: %w", err)
+		if errors.Is(err, fs.ErrNotExist) {
+			return fmt.Errorf("failed to show trust policy as the trust policy file does not exist.\nYou can import one using `notation policy import <path-to-policy.json>`")
+		}
+		return fmt.Errorf("failed to show trust policy: %w", err)
 	}
 	var doc trustpolicy.Document
 	if err = json.Unmarshal(policyJSON, &doc); err == nil {

test/e2e/suite/command/policy.go

@@ -30,7 +30,17 @@ var _ = Describe("trust policy maintainer", func() {
 			Host(Opts(), func(notation *utils.ExecOpts, artifact *Artifact, vhost *utils.VirtualHost) {
 				notation.ExpectFailure().
 					Exec("policy", "show").
-					MatchErrKeyWords("failed to load trust policy configuration", "notation policy import")
+					MatchErrKeyWords("failed to show trust policy", "notation policy import")
+			})
+		})
+
+		It("should show error and hint if policy without read permission", func() {
+			Host(Opts(AddTrustPolicyOption(TrustPolicyName)), func(notation *utils.ExecOpts, artifact *Artifact, vhost *utils.VirtualHost) {
+				trustPolicyPath := vhost.AbsolutePath(NotationDirName, TrustPolicyName)
+				os.Chmod(trustPolicyPath, 0200)
+				notation.ExpectFailure().
+					Exec("policy", "show").
+					MatchErrKeyWords("failed to show trust policy", "permission denied")
 			})
 		})
 
@@ -60,7 +70,17 @@ var _ = Describe("trust policy maintainer", func() {
 		It("should fail if no file path is provided", func() {
 			Host(opts, func(notation *utils.ExecOpts, artifact *Artifact, vhost *utils.VirtualHost) {
 				notation.ExpectFailure().
-					Exec("policy", "import")
+					Exec("policy", "import").
+					MatchErrKeyWords("requires 1 argument but received 0")
+
+			})
+		})
+
+		It("should fail if more than one file path is provided", func() {
+			Host(opts, func(notation *utils.ExecOpts, artifact *Artifact, vhost *utils.VirtualHost) {
+				notation.ExpectFailure().
+					Exec("policy", "import", "a", "b").
+					MatchErrKeyWords("requires 1 argument but received 2")
 			})
 		})