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/swift/swift.go

@@ -311,11 +311,8 @@ func (be *beSwift) removeKeys(ctx context.Context, t restic.FileType) error {
 
 // IsNotExist returns true if the error is caused by a not existing file.
 func (be *beSwift) IsNotExist(err error) bool {
-	if e, ok := errors.Cause(err).(*swift.Error); ok {
-		return e.StatusCode == http.StatusNotFound
-	}
-
-	return false
+	var e *swift.Error
+	return errors.As(err, &e) && e.StatusCode == http.StatusNotFound
 }
 
 // Delete removes all restic objects in the container.