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.

vfs/vfscache/writeback/writeback.go

@@ -5,12 +5,12 @@ package writeback
 import (
 	"container/heap"
 	"context"
+	"errors"
 	"sync"
 	"sync/atomic"
 	"time"
 
 	"github.com/rclone/rclone/fs"
-	"github.com/rclone/rclone/fs/fserrors"
 	"github.com/rclone/rclone/vfs/vfscommon"
 )
 
@@ -365,7 +365,7 @@ func (wb *WriteBack) upload(ctx context.Context, wbItem *writeBackItem) {
 		if wbItem.delay > maxUploadDelay {
 			wbItem.delay = maxUploadDelay
 		}
-		if _, uerr := fserrors.Cause(err); uerr == context.Canceled {
+		if errors.Is(err, context.Canceled) {
 			fs.Infof(wbItem.name, "vfs cache: upload canceled")
 			// Upload was cancelled so reset timer
 			wbItem.delay = wb.opt.WriteBack