Export - warn but don't block exporting duplicate managed entity

[colemanw]

Per discussion, sometimes there's a legit reason to export a managed entity that already exists.

The use-case here is that Afform autogenerates a managed entity for navigation items that belong to the form, but civix exports them as managed entities in their own right. This isn't a real conflict but civix thinks it is.

[totten]

civibot, test this please

[totten]

Test failure unrelated