No upstream version found doesn't mean there is error in project check

[Zlopez]

Description

Currently if we don't retrieve any version we raise an exception, but this could be correct if there is no new version (at least for GitHub, which is using cursor now). There should be difference between no new version found and no version found at all (project is incorrectly setup or the upstream doesn't exist).

Requirements

• If project could be checked, but no version could be retrieved it shouldn't throw any error. (This is currently happening only in GitHub backend)

How this will make Anitya better

It will prevent raising false alarms about no longer updated projects.