all: Move away from pkg/errors, easy cases

github.com/pkg/errors is no longer getting updates, because Go 1.13
went with the more flexible errors.{As,Is} function. Use those instead:
errors from pkg/errors already support the Unwrap interface used by 1.13
error handling. Also:

* check for io.EOF with a straight ==. That value should not be wrapped,
  and the chunker (whose error is checked in the cases changed) does not
  wrap it.
* Give custom Error methods pointer receivers, so there's no ambiguity
  when type-switching since the value type will no longer implement error.
* Make restic.ErrAlreadyLocked private, and rename it to
  alreadyLockedError to match the stdlib convention that error type
  names end in Error.
* Same with rest.ErrIsNotExist => rest.notExistError.
* Make s3.Backend.IsAccessDenied a private function.

internal/backend/sftp/sftp.go

@@ -183,7 +183,6 @@ func (r *SFTP) ReadDir(ctx context.Context, dir string) ([]os.FileInfo, error) {
 
 // IsNotExist returns true if the error is caused by a not existing file.
 func (r *SFTP) IsNotExist(err error) bool {
-	err = errors.Cause(err)
 	return errors.Is(err, os.ErrNotExist)
 }
 
@@ -496,7 +495,7 @@ func (r *SFTP) Test(ctx context.Context, h restic.Handle) (bool, error) {
 	defer r.sem.ReleaseToken()
 
 	_, err := r.c.Lstat(r.Filename(h))
-	if os.IsNotExist(errors.Cause(err)) {
+	if errors.Is(err, os.ErrNotExist) {
 		return false, nil
 	}