ENH: wrap to_dict() tracebacks for more user-friendly errors

[jakevdp]

This does two things:

• adds a custom exception that wraps jsonschema.ValidationError and provides tracebacks that are more useful for Altair specifications

• adds a validate='deep' option that triggers any time there is a validation error on a top-level object.

The result is friendlier tracebacks; for example (compare to #480):

import altair as alt
alt.Chart()

---------------------------------------------------------------------------
SchemaValidationError                     Traceback (most recent call last)
~/anaconda/envs/vega3/lib/python3.6/site-packages/IPython/core/formatters.py in __call__(self, obj, include, exclude)
    968 
    969             if method is not None:
--> 970                 return method(include=include, exclude=exclude)
    971             return None
    972         else:

~/github/altair-viz/altair/altair/vegalite/v2/api.py in _repr_mimebundle_(self, include, exclude)
    239     def _repr_mimebundle_(self, include, exclude):
    240         """Return a MIME bundle for display in Jupyter frontends."""
--> 241         return renderers.get()(self.to_dict())
    242 
    243     def repeat(self, row=Undefined, column=Undefined, **kwargs):

~/github/altair-viz/altair/altair/vegalite/v2/api.py in to_dict(self, *args, **kwargs)
    212         if dct is None:
    213             kwargs['validate'] = 'deep'
--> 214             dct = super(TopLevelMixin, copy).to_dict(*args, **kwargs)
    215 
    216         if is_top_level:

~/github/altair-viz/altair/altair/utils/schemapi.py in to_dict(self, validate, ignore, context)
    225                 self.validate(result)
    226             except jsonschema.ValidationError as err:
--> 227                 raise SchemaValidationError(self, err)
    228         return result
    229 

SchemaValidationError: Invalid specification

        altair.vegalite.v2.api.Chart, validating 'required'

        'mark' is a required property

[jakevdp]

Tests pass, I'm going to merge!