Test that pasting a deleted item doesn't give an error.

When pasting an item that has been deleted by someone else
since it was cut or copied, we should not get an error.

https://dev.plone.org/ticket/13577
Catch CopyErrors for Item Not Found.

Display a friendly string instead.
(https://dev.plone.org/ticket/13577)
Use str(e) to read error string without authorization issues.
If it's not an Item Not Found error, do what would have happened otherwise.
Deduplicate code: Move fallback action into method.