fix: ensure proper usage of DataDir (#12300)

- GetObject() should always use a common dataDir to
  read from when it starts reading, this allows the
  code in erasure decoding to have sane expectations.

- Healing should always heal on the common dataDir, this
  allows the code in dangling object detection to purge
  dangling content.

These both situations can happen under certain types of
retries during PUT when server is restarting etc, some
namespace entries might be left over.

cmd/api-errors.go

@@ -766,7 +766,7 @@ var errorCodes = errorCodeMap{
 	},
 	ErrSlowDown: {
 		Code:           "SlowDown",
-		Description:    "Please reduce your request",
+		Description:    "Resource requested is unreadable, please reduce your request rate",
 		HTTPStatusCode: http.StatusServiceUnavailable,
 	},
 	ErrInvalidPrefixMarker: {