error parsing image manifest on unsuccessful builds

[andrewmackett]

Unsuccessful Cloud Builds that have the images field defined create an error in the BigQuery notifier:

E1114 12:05:43.832522       1 notifiers.go:556] failed to run SendNotification: error parsing image manifest: error obtaining image reference: GET https://gcr.io/v2/project-id/image/manifests/tag: MANIFEST_UNKNOWN: Failed to fetch "tag"

I wouldn't expect the image to have been pushed to the repository in an unsuccessful build, so there will be no manifest for the notifier to lookup, causing the error.

I've opened #211 with a simple fix. This change executes the build image logic only if the build has a SUCCESS status.