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 think maybe a better instruction at the end would be 'Please run `berks update` to update Berksfile.lock.'  That will either get them a working Berksfile.lock if it can be done and keep them closer to their original, or it will give an error equally meaningful as `berks install` if it can't.  Any object to that suggested course of action?