add step to validate preprocessors that relies on nbformat 4.4

[mpacer]

This PR will break not pass on travis until jupyter/nbformat#103 is changed.

We had thought (in our discussion in #643) that preprocessors currently would not be able to violate the NotebookNode format… turns out, that isn't the case!

Lo, behold the PizzaPreprocessor:

class PizzaPreprocessor(Preprocessor):
    """Simple preprocessor that adds a 'pizza' entry to the NotebookNode.  Used 
    to test Exporter.
    """

    def preprocess(self, nb, resources):
        nb['pizza'] = 'cheese'
        return nb, resources

which is part of our TestExporter tests and is almost 4 years old.

This is definitely not a valid attribute, so it seems we've not been strict about enforcing the "nb→nb" aspect of Preprocessors for a while.

This change would enforce the constraint that @minrk suggested in #643, that we allow only additional attributes to be added to the intermediate representation of the notebook.

That would mean that the PizzaPreprocessor would now be ok (at least as far as the intermediate representation goes), as well as the cell.transient field used in #643.

It also would enforce this constraint on 3rd-party extensions relying on the native preprocessor support in the core Exporter (which is probably going to break some extensions but we probably want to know what extensions actually cause non-valid notebooks to be written).

[takluyver]

I wonder if it's worth catching the validation error and re-raising with a message saying which preprocessor made it invalid?

[mpacer]

That is surprisingly non-trivial to do given the way that NotebookValidationError is written.

[mpacer]

see jupyter/nbformat#105 for more on why it's nontrivial.

[takluyver]

I've posted the same idea on that nbformat issue, but for easy reference, this is what I'd suggest:

try: 
    nbformat.validate(nbc, relax_add_props=True)
except nbformat.ValidationError:
    self.log.error('Notebook is invalid after preprocessor %r', preprocessor)
    raise  # With no args, raise re-raises the same exception

It would be neater to have it in the exception message itself, but I think having it in logging is good enough.

[mpacer]

Ok should be good to go now once nbformat is released

[mpacer]

Reran CI now that nbformat 4.4 is released and it passes. This should be good to merge.