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.

lib/errors/errors.go

@@ -1,22 +1,9 @@
 package errors
 
 import (
-	"errors"
-	"fmt"
 	"reflect"
 )
 
-// New returns an error that formats as the given text.
-func New(text string) error {
-	return errors.New(text)
-}
-
-// Errorf formats according to a format specifier and returns the string
-// as a value that satisfies error.
-func Errorf(format string, a ...interface{}) error {
-	return fmt.Errorf(format, a...)
-}
-
 // WalkFunc is the signature of the Walk callback function. The function gets the
 // current error in the chain and should return true if the chain processing
 // should be aborted.
@@ -27,7 +14,6 @@ type WalkFunc func(error) bool
 // is stopped and no further calls will be made.
 //
 // The next error in the chain is determined by the following rules:
-// - If the current error has a `Cause() error` method (github.com/pkg/errors),
 //   the return value of this method is used.
 // - If the current error has a `Unwrap() error` method (golang.org/x/xerrors),
 //   the return value of this method is used.