Rollback API is returning an error

[zachaller]

The rollback api via UI and CLI is returning an error but still rolls back to the previous version.

{
  "code": 2,
  "message": "rollout.argoproj.io \"istio-host-split\" not found"
}

To reproduce go into the UI and try to rollback while looking at the browsers network console, via the CLI the CLI returns the error

[nikhil1raghav]

This error comes while calling getRollout after undoRollout. Cache of the informer is not being synced before using it. @zachaller can you assign this to me I'll raise a PR for the fix.