[native] move alert to effect where we call getSIWENonceCall()

Summary: when disconnected from the keyserver, this alert was being displayed twice on the second failed attempt to get the nonce and beyond

Test Plan: when connected to local keyserver, the get nonce call still succeeds. when disconnected, the alert is only displayed once

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10896