Added some error logging to dataProvider
dataProvider now presents dummy transcriptions.

- A dummy transcription on the client side now exists as two cases:
  1: A models/Transcription, that has isDummy() === true
  2: An instance of models/DummyTranscription created when no Transcription was found by the TranscriptionMap.
- I've also added TranscriptionMap.getDummies() as an easy way to get all currently known dummies.