Missing translations for constraints

Some translations are missing on localize.drupal.org and can't be translated, for example:

• UserNameUnique: The username %name is already taken.
• UserMailUnique: The email address %value is already taken.

See https://localize.drupal.org/translate/languages/nl/translate?project=dru...

POTX can extract them but then the filename has to end with Constraint and the message variable has to be named "%message":

  if (substr($name_parts['filename'], -10) == 'Constraint' && $api_version > POTX_API_7) {
    $constraint_extract = TRUE;
  }

Proposed solution:

I suggest to rename the constraints in core to end with "Constraint".
Alternative solution would be to test if the class extends "Constraint".