Confirm client is found and authenticated before "yield next"

This PR relates to issue: https://github.com/jembi/openhim-core-js/issues/281
Added in a check to make sure the client is found and "this.authenticated" isn't null before moving on, otherwise client is not authenticated
CN stands for Common Name. So perhaps rather say "Certificate Authentication Failed: the certificate's common name did not match any client's domain attribute."