a8n: Don't create changeset when ID is invalid

Created by: eseliger

Steps to reproduce:

1. Go to campaigns
2. Add a changeset manually, using a not existing ID
3. See dev stack log output

Expected behavior:

The error is thrown and it's forgotten about.

Actual behavior:

The changeset still get's created and is getting synced over and over again, logging messages like the below:

repo-updater | ERROR PreSync, error: error in GraphQL response: Could not resolve to a PullRequest with the number of 65526.