function hook_form_validate($form, &$form_state) {
  $errors = form_get_errors();

  if ($errors) {
    // Clear errors.
    form_clear_error();
    // Clear error messages.
    $error_messages = drupal_get_messages('error');
    // Initialize an array where removed error messages are stored.
    $removed_messages = array();

    // Remove all errors originated by the 'foo][bar' element.
    foreach ($errors as $name => $error_message) {
      if ($name == 'body][und][0][value') {
        $removed_messages[] = $error_message;
        unset($errors[$name]);
      }
    }

    // Reinstate remaining errors.
    foreach ($errors as $name => $error) {
      form_set_error($name, $error);
      // form_set_error() calls drupal_set_message(), so we have to filter out
      // these from the error messages as well.
      $removed_messages[] = $error;
    }

    // Reinstate remaining error messages (which, at this point, are messages that
    // were originated outside of the validation process).
    foreach (array_diff($error_messages['error'], $removed_messages) as $message) {
      drupal_set_message($message, 'error');
    }
  }
}

Source: http://goo.gl/nI55mb