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/ncdu/ncdu.go

@@ -16,7 +16,6 @@ import (
 	"github.com/atotto/clipboard"
 	runewidth "github.com/mattn/go-runewidth"
 	termbox "github.com/nsf/termbox-go"
-	"github.com/pkg/errors"
 	"github.com/rclone/rclone/cmd"
 	"github.com/rclone/rclone/cmd/ncdu/scan"
 	"github.com/rclone/rclone/fs"
@@ -314,7 +313,7 @@ func (u *UI) Draw() error {
 	// Plot
 	err := termbox.Clear(termbox.ColorDefault, termbox.ColorDefault)
 	if err != nil {
-		return errors.Wrap(err, "failed to clear screen")
+		return fmt.Errorf("failed to clear screen: %w", err)
 	}
 
 	// Header line
@@ -432,7 +431,7 @@ func (u *UI) Draw() error {
 	}
 	err = termbox.Flush()
 	if err != nil {
-		return errors.Wrap(err, "failed to flush screen")
+		return fmt.Errorf("failed to flush screen: %w", err)
 	}
 	return nil
 }
@@ -742,7 +741,7 @@ func NewUI(f fs.Fs) *UI {
 func (u *UI) Show() error {
 	err := termbox.Init()
 	if err != nil {
-		return errors.Wrap(err, "termbox init")
+		return fmt.Errorf("termbox init: %w", err)
 	}
 	defer termbox.Close()
 
@@ -766,7 +765,7 @@ outer:
 		//Reset()
 		err := u.Draw()
 		if err != nil {
-			return errors.Wrap(err, "draw failed")
+			return fmt.Errorf("draw failed: %w", err)
 		}
 		var root *scan.Dir
 		select {
@@ -775,7 +774,7 @@ outer:
 			u.setCurrentDir(root)
 		case err := <-errChan:
 			if err != nil {
-				return errors.Wrap(err, "ncdu directory listing")
+				return fmt.Errorf("ncdu directory listing: %w", err)
 			}
 			u.listing = false
 		case <-updated:

cmd/ncdu/scan/scan.go

@@ -3,10 +3,10 @@ package scan
 
 import (
 	"context"
+	"fmt"
 	"path"
 	"sync"
 
-	"github.com/pkg/errors"
 	"github.com/rclone/rclone/fs"
 	"github.com/rclone/rclone/fs/walk"
 )
@@ -185,7 +185,7 @@ func Scan(ctx context.Context, f fs.Fs) (chan *Dir, chan error, chan struct{}) {
 				var ok bool
 				parent, ok = parents[parentPath]
 				if !ok {
-					errChan <- errors.Errorf("couldn't find parent for %q", dirPath)
+					errChan <- fmt.Errorf("couldn't find parent for %q", dirPath)
 				}
 			}
 			d := newDir(parent, dirPath, entries, err)
@@ -202,7 +202,7 @@ func Scan(ctx context.Context, f fs.Fs) (chan *Dir, chan error, chan struct{}) {
 			return nil
 		})
 		if err != nil {
-			errChan <- errors.Wrap(err, "ncdu listing failed")
+			errChan <- fmt.Errorf("ncdu listing failed: %w", err)
 		}
 		errChan <- nil
 	}()