Notify commercial team if membership numbers fail to generate

Higher-level members are created via capsuleCRM, not via the local signup. However, these *must* have an email entered. CapsuleCRM cannot validate this for us, so it means that we need to provide some feedback if the email is not entered correctly. Requires theodi/open-orgn-services#257 first. Do not merge until I give the OK here, this needs a bit of synchronisation.