API.Membership.delete fatals on invalid id

Normally on the api we don't pre-validate ids where sql will give an error (which we can interpret if they are incorrect for performance reasons)

However http://issues.civicrm.org/jira/browse/CRM-12147 introduced a pre-delete lookup which causes a fatal on invalid data.

(Note that we have in other cases advised the intergrator that they should do the lookup themselves for performance reasons on the pre hook & stash the value data in their code - I'm not necessarily adverse to us doing the lookup on delete but we should do it consistently)