Remove github.com/pkg/errors and replace with std library version

This is possible now that we no longer support go1.12 and brings
rclone into line with standard practices in the Go world.

This also removes errors.New and errors.Errorf from lib/errors and
prefers the stdlib errors package over lib/errors.

cmd/serve/sftp/server.go

@@ -15,6 +15,7 @@ import (
 	"crypto/x509"
 	"encoding/base64"
 	"encoding/pem"
+	"errors"
 	"fmt"
 	"io/ioutil"
 	"net"
@@ -22,7 +23,6 @@ import (
 	"path/filepath"
 	"strings"
 
-	"github.com/pkg/errors"
 	"github.com/rclone/rclone/cmd/serve/proxy"
 	"github.com/rclone/rclone/cmd/serve/proxy/proxyflags"
 	"github.com/rclone/rclone/fs"
@@ -237,7 +237,7 @@ func (s *server) serve() (err error) {
 			// If loading a cached key failed, make the keys and retry
 			err = file.MkdirAll(cachePath, 0700)
 			if err != nil {
-				return errors.Wrap(err, "failed to create cache path")
+				return fmt.Errorf("failed to create cache path: %w", err)
 			}
 			if strings.HasSuffix(keyPath, string(os.PathSeparator)+"id_rsa") {
 				const bits = 2048
@@ -250,10 +250,10 @@ func (s *server) serve() (err error) {
 				fs.Logf(nil, "Generating Ed25519 key pair at %q", keyPath)
 				err = makeEd25519SSHKeyPair(keyPath+".pub", keyPath)
 			} else {
-				return errors.Errorf("don't know how to generate key pair %q", keyPath)
+				return fmt.Errorf("don't know how to generate key pair %q", keyPath)
 			}
 			if err != nil {
-				return errors.Wrap(err, "failed to create SSH key pair")
+				return fmt.Errorf("failed to create SSH key pair: %w", err)
 			}
 			// reload the new key
 			private, err = loadPrivateKey(keyPath)
@@ -270,7 +270,7 @@ func (s *server) serve() (err error) {
 	// accepted.
 	s.listener, err = net.Listen("tcp", s.opt.ListenAddr)
 	if err != nil {
-		return errors.Wrap(err, "failed to listen for connection")
+		return fmt.Errorf("failed to listen for connection: %w", err)
 	}
 	fs.Logf(nil, "SFTP server listening on %v\n", s.listener.Addr())
 
@@ -313,11 +313,11 @@ func (s *server) Close() {
 func loadPrivateKey(keyPath string) (ssh.Signer, error) {
 	privateBytes, err := ioutil.ReadFile(keyPath)
 	if err != nil {
-		return nil, errors.Wrap(err, "failed to load private key")
+		return nil, fmt.Errorf("failed to load private key: %w", err)
 	}
 	private, err := ssh.ParsePrivateKey(privateBytes)
 	if err != nil {
-		return nil, errors.Wrap(err, "failed to parse private key")
+		return nil, fmt.Errorf("failed to parse private key: %w", err)
 	}
 	return private, nil
 }
@@ -328,13 +328,13 @@ func loadPrivateKey(keyPath string) (ssh.Signer, error) {
 func loadAuthorizedKeys(authorizedKeysPath string) (authorizedKeysMap map[string]struct{}, err error) {
 	authorizedKeysBytes, err := ioutil.ReadFile(authorizedKeysPath)
 	if err != nil {
-		return nil, errors.Wrap(err, "failed to load authorized keys")
+		return nil, fmt.Errorf("failed to load authorized keys: %w", err)
 	}
 	authorizedKeysMap = make(map[string]struct{})
 	for len(authorizedKeysBytes) > 0 {
 		pubKey, _, _, rest, err := ssh.ParseAuthorizedKey(authorizedKeysBytes)
 		if err != nil {
-			return nil, errors.Wrap(err, "failed to parse authorized keys")
+			return nil, fmt.Errorf("failed to parse authorized keys: %w", err)
 		}
 		authorizedKeysMap[string(pubKey.Marshal())] = struct{}{}
 		authorizedKeysBytes = bytes.TrimSpace(rest)