membership status delete fails silently when membership exists

This is causing silent failure (it was in membership test - I fixed those but created a test for this problem:

api_v3_MembershipStatusTest::testDeleteWithMembershipError

instead of returning an error message it does a redirect thing. I tried adding skipRedirect to the membership_status_delete function but I must have my brain on backwards 'cos it's not working so can someone else look.