Improve error message when lockfile has missing cookbook or version.

This was found in #1221 which had a corrupted Berksfile.lock due to both metadata.rb and metadata.json present.  However, this could also happen if someone deleted a cookbook from a server, or if someone changed the sources in Berksfile and tried to do an install.  The previous behavior was the ugly exception trace in #1221; now it generates a message explaining that either the cookbook is missing or the version and offers advice on how to fix.
I don't really see a need to separate these two edge cases. I would rather wrap everything up in a single exception. I think the additional logic is superfluous technical debt and the information does not actually benefit the user anymore than the example exception I supplied above. It's also requires an additional API call just to display an error message.