Added plotly/exceptions.py and plotly/utils.py

plotly/exceptions.py
--------------------

This contains all of the custom exceptions we want to use in the plotly
package. A base `PlotlyError` subclasses from `Exception` and all other
errors subclass from `PlotlyError`.

plotly/utils.py
---------------

This is a blank file, intended to be populated with functionality from
plotly.py.