Handle AR exceptions when a new Catalog item is created

A call to create_request might lead to AR exceptions in the model that
need to be rescued and passed on to the controller.

https://bugzilla.redhat.com/show_bug.cgi?id=1095457
Merge pull request #468 from AparnaKarve/bz1095457_invalid_request_add_catalog_item

Handle AR exceptions gracefully when a new Catalog item is created