Add missing reponse functions

The `etag` decorator was added in a595665ab75c5995f0cb3af7463215f9cd7aabf7, following Miguel Grinberg's [`decorators` module](https://github.com/miguelgrinberg/api-pycon2014/blob/master/api/decorators.py), but the referenced [`errors` module](https://github.com/miguelgrinberg/api-pycon2014/blob/master/api/errors.py) was forgotten, causing runtime failures:

    Traceback (most recent call last):
      File "/app/.heroku/python/lib/python2.7/site-packages/gunicorn/arbiter.py", line 495, in spawn_worker
        worker.init_process()
      File "/app/.heroku/python/lib/python2.7/site-packages/gunicorn/workers/base.py", line 106, in init_process
        self.wsgi = self.app.wsgi()
      File "/app/.heroku/python/lib/python2.7/site-packages/gunicorn/app/base.py", line 114, in wsgi
        self.callable = self.load()
      File "/app/.heroku/python/lib/python2.7/site-packages/gunicorn/app/wsgiapp.py", line 62, in load
      File "/app/.heroku/python/lib/python2.7/site-packages/gunicorn/app/wsgiapp.py", line 49, in load_wsgiapp
        return self.load_wsgiapp()
        return util.import_app(self.app_uri)
      File "/app/.heroku/python/lib/python2.7/site-packages/gunicorn/util.py", line 354, in import_app
        __import__(module)
      File "/app/sandman-on-heroku.py", line 2, in <module>
        from sandman import app
      File "/app/.heroku/python/lib/python2.7/site-packages/sandman/__init__.py", line 15, in <module>
        from . import sandman
        from .decorators import etag
      File "/app/.heroku/python/lib/python2.7/site-packages/sandman/decorators.py", line 4, in <module>
        from .errors import precondition_failed, not_modified
    ImportError: No module named errors

The reference to `errors` was removed in 0ada6f19b416793661f102909808dd7b455d2b04, without fixing the problem.

This adds the missing response functions.