improve depsolver error message

- at least return the last cookbook that failed inside the recursive
  bowels of the depsolver.  not sure if this will produce a slightly
  misleading error messge in the case of a cookbook resolution failure,
  but it should give at least point at a cookbook that has a problem.
slight better error message on constraint failure
Merge pull request #40 from opscode/lcg/better-depsolver-error

improve depsolver error message