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/settier/settier.go

@@ -2,8 +2,8 @@ package settier
 
 import (
 	"context"
+	"fmt"
 
-	"github.com/pkg/errors"
 	"github.com/rclone/rclone/cmd"
 	"github.com/rclone/rclone/fs/operations"
 	"github.com/spf13/cobra"
@@ -47,7 +47,7 @@ Or just provide remote directory and all files in directory will be tiered
 		cmd.Run(false, false, command, func() error {
 			isSupported := fsrc.Features().SetTier
 			if !isSupported {
-				return errors.Errorf("Remote %s does not support settier", fsrc.Name())
+				return fmt.Errorf("Remote %s does not support settier", fsrc.Name())
 			}
 
 			return operations.SetTier(context.Background(), fsrc, tier)