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.

fs/cutoffmode.go

@@ -3,8 +3,6 @@ package fs
 import (
 	"fmt"
 	"strings"
-
-	"github.com/pkg/errors"
 )
 
 // CutoffMode describes the possible delete modes in the config
@@ -40,7 +38,7 @@ func (m *CutoffMode) Set(s string) error {
 			return nil
 		}
 	}
-	return errors.Errorf("Unknown cutoff mode %q", s)
+	return fmt.Errorf("Unknown cutoff mode %q", s)
 }
 
 // Type of the value
@@ -52,7 +50,7 @@ func (m *CutoffMode) Type() string {
 func (m *CutoffMode) UnmarshalJSON(in []byte) error {
 	return UnmarshalJSONFlag(in, m, func(i int64) error {
 		if i < 0 || i >= int64(len(cutoffModeToString)) {
-			return errors.Errorf("Out of range cutoff mode %d", i)
+			return fmt.Errorf("Out of range cutoff mode %d", i)
 		}
 		*m = (CutoffMode)(i)
 		return nil