release.latestDownloaded -> latestKnown, errors

Since 0.9.0 it hasn't had anything to do about anything being
downloaded, just metadata, so the new name is better.

And without auto-retry, there's no need for this to have anything to do
with buildmessage.

Add a bit more support for the very unlikely case that you're running
meteor outside an app and the catalog has no information about any
METEOR@ release.