[ready] fix subdoc validation error paths

[aheckmann]

subdocument validation error paths were reported incorrectly.

var plan = new Schema({ title: { type: 'string', required: true }})
var room = new Schema({ lessonplans: [plan] });
...
var r = new Room({ lessonplans: [{ title: '' }] });
r.save(function (err) {
  // old incorrect behavior
  console.log(err.errors.title) // Validator "required" ...
  // corrected behavior
  console.log(err.errors['lessonplans.0.title']) // Validator "required" ...
})

closes #725