Make Delete User failure overridable

Make Delete User failure overridable

Make mrpeter's new ACTOR_USER_DELETE_OPEN_XACTS event overridable to preserve
existing functionality (the delete user code will wipe out all transactions if
allowed to, and will leave copy statuses as "Checked Out" if needed, which is
okay in EG even if there aren't corresponding circ records--some libraries
migrate data like this if the information available is sparse).

So if you want to be able to do this without a super user, you'll need to create
an ACTOR_USER_DELETE_OPEN_XACTS.override perm and assign it to perm groups
and/or users.

Signed-off-by: Jason Etheridge <jason@esilibrary.com>