CAPX-58: Added validation to importer.

CAPX-58: Added validation to importer.

How to test:
* Create new content type
* Create new mapper with this content type
* Create 2+ importers with this mapper, do not import
* Remove content type created in the first step
* Click "Update profiles now" on importers page

Please change to:

$message_text .= t("The following importers are using an invalid mapper. Please update or delete the mapper settings: !importers);