Improve error message when lock file 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.