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 this should just be:

```ruby
source = berksfile.source_for(name, version)

# Some comment
raise InvalidLockfileCookbook if source.nil?
```