Handle corner case in getLatest artifact, which cascades to Poller

[brewkode]

In our setup, we've noticed that getObjectMetadata request (after doing a getLatest) fails with 404. One possible code path that leads to this is getLatest returning Revisions.base(), which is problematic, since, we consider it to be a valid revision and try to do a metadata fetch. Added a fix to handle it with some logging enabled on the plug-in side.

[brewkode]

@ashwanthkumar @manojlds Please take a look at this and share your feedback