Dataset cloud function preprocessing should handle improper datasets.

[harrykeightley]

Currently the preprocessing workflow blindly attempts to create processing jobs from the files supplied in a dataset.
This would fail if the user selected improper files, not enough files etc.

Instead, after forming a dataset object from the incoming event, the process_dataset function should check dataset validity before batching and pushing jobs to the pubsub queue.

If the dataset is found to be invalid, we should indicate so on the dataset model in firestore (setting status to error or something).