Improve error message when the "label" column is missing in the dataset

[dcfidalgo]

At the moment, if the dataset is missing the "label" column (or whatever column is necessary to train the model) and you want to train the model with it, the error message is:

RuntimeError: The model you are trying to optimize does not contain a 'loss' key in the output of model.forward(inputs).

I think we should catch this failure earlier and print out a more precise error message.

One idea would be to have a bool argument for_training in the Dataset.to_instance method. Depending on this argument, it checks for the necessary columns.
Edit: This idea is actually bull**** since we only call to_instance when we want to create the vocab or train the pipeline ...