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/rclone/internal_test.go

@@ -18,7 +18,8 @@ func TestRcloneExit(t *testing.T) {
 	cfg := NewConfig()
 	cfg.Remote = dir
 	be, err := Open(cfg, nil)
-	if e, ok := errors.Cause(err).(*exec.Error); ok && e.Err == exec.ErrNotFound {
+	var e *exec.Error
+	if errors.As(err, &e) && e.Err == exec.ErrNotFound {
 		t.Skipf("program %q not found", e.Name)
 		return
 	}