FIX Use valid class for model importers

[GuySartorelli]

Description

BulkLoader is being initialised with the tab name instead of the class name when a custom tab name is provided.
This is related to the wider issue #1364

Issues

• Broken builds #1656

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[GuySartorelli]

false is not a valid return type for this method in Silverstripe CMS 5.
It causes a behat failure in various scenarios of the "Import in GridField" behat feature:

Uncaught TypeError: SilverStripe\Admin\ModelAdmin::import(): Return value must be of type SilverStripe\Control\HTTPResponse, bool returned

[michalkleiner]

👍 MOG

[GuySartorelli]

The failing behat test will be resolved separately - it's a pre-existing failure not related to this PR.