AttributeError hides an easy way to break "values" sheet

[eyeseast]

Here: https://github.com/tarbell-project/tarbell/blob/master/tarbell/app.py#L526

To reproduce, start a project with a values sheet, but delete the key column header. The project will suddenly have no data, just a sad {}. This is because copy_global_values (https://github.com/tarbell-project/tarbell/blob/master/tarbell/app.py#L537) looks for a key column, can't find it and throws an attribute error. So _get_context_from_gdoc returns an empty dictionary.