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.
It does because `MissingLockfileCookbookVersion` is a subclass of `CookbookNotFound` per the CR feedback above.  That method redefine's the singleton method `status_code` from the setter helper in the base class to an accessor method to get at the status code; so when you get to here https://github.com/ppgengler/berkshelf/blob/master/lib/berkshelf/errors.rb#L484, if the name wasn't changed its trying to call an accessor now with a parameter (instead of the BerkshelfError's helper method) and fails.