Rescue CookbookNotFound in lockfile#load!

It's possible that a source is locked that contains a path location, and that path location was renamed or no longer exists. When loading the lockfile, Berkshelf will throw an error if it can't find a cookbook that previously existed at a path location.

This was never an issue before, because the lockfile was not trusted if the SHAs did not match. Now that we always evaluate, it's possible for the lockfile to contain stale entries.