Clean up non-terminal exception

[cmc333333]

I think this occurs when requesting a layer that returns a 404.

Traceback (most recent call last):
  File "/opt/python/3.5.0/lib/python3.5/threading.py", line 923, in _bootstrap_inner
    self.run()
  File "/opt/python/3.5.0/lib/python3.5/threading.py", line 871, in run
    self._target(*self._args, **self._kwargs)
  File "/home/travis/build/eregs/regulations-site/regulations/generator/generator.py", line 60, in one_layer
    version)
  File "/home/travis/build/eregs/regulations-site/regulations/generator/generator.py", line 107, in get_layer_json
    layer_json = dict(newer_layer)  # copy
TypeError: 'NoneType' object is not iterable