Improve error messages when a platform being upgraded is not supported by upgrade image

[jgallag88]

Following #166, there is a new upgrade failure mode. In particular, we support building upgrade images which don't contain all of the packages needed to run upgrade on any supported platform. This is done to make it easier to iterate on changes during development, but could lead to a confusing situation for those testing upgrade internally. We should add an explicit check for this situation and provide guidance on how to proceed when this situation is encountered.

[pzakha]

@jgallag88 I think a better place for this type of issue would be JIRA since this kind of check wouldn't be performed by appliance-build. I know this is old, but I do believe this still applies. I'm trying to close out old github issues that are not relevant anymore.