Lack of memcached oopses login

[Simon Davy]

If memached is unavailable for any reason, login fails in the rate limiting code.

It would be better if sso handled this more gracefully, albiet with degraded functionality.

ValueError "Key ':1:rl-86.10.217.127-de0da8968b776ff204592d442ac793a37abeef2f-201410030918' not found"
  File "/srv/login.staging.ubuntu.com/staging/code/r1190/lib/django/core/handlers/base.py", line 112, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/srv/login.staging.ubuntu.com/staging/code/r1190/lib/django/views/generic/base.py", line 69, in view
    return self.dispatch(request, *args, **kwargs)
  File "/srv/login.staging.ubuntu.com/staging/code/r1190/lib/django/utils/decorators.py", line 29, in _wrapper
    return bound_func(*args, **kwargs)
  File "/srv/login.staging.ubuntu.com/staging/code/r1190/src/webui/decorators.py", line 52, in _redirect_home_if_logged_in
    return func(request, *args, **kwargs)
  File "/srv/login.staging.ubuntu.com/staging/code/r1190/lib/django/utils/decorators.py", line 25, in bound_func
    return func(self, *args2, **kwargs2)
  File "/srv/login.staging.ubuntu.com/staging/code/r1190/lib/django/utils/decorators.py", line 29, in _wrapper
    return bound_func(*args, **kwargs)
  File "/srv/login.staging.ubuntu.com/staging/code/r1190/src/webui/decorators.py", line 58, in _dont_cache_decorator
    response = func(request, *args, **kwargs)
  File "/srv/login.staging.ubuntu.com/staging/code/r1190/lib/django/utils/decorators.py", line 25, in bound_func
    return func(self, *args2, **kwargs2)
  File "/srv/login.staging.ubuntu.com/staging/code/r1190/lib/django/utils/decorators.py", line 29, in _wrapper
    return bound_func(*args, **kwargs)
  File "/srv/login.staging.ubuntu.com/staging/code/r1190/src/webui/decorators.py", line 198, in wrapper
    return self.view_wrapper(request, fn, *args, **kwargs)
  File "/srv/login.staging.ubuntu.com/staging/code/r1190/src/webui/decorators.py", line 208, in view_wrapper
    self.cache_incr(self.current_key(request))
  File "/srv/login.staging.ubuntu.com/staging/code/r1190/src/webui/decorators.py", line 226, in cache_incr
    cache.incr(key)
  File "/srv/login.staging.ubuntu.com/staging/code/r1190/lib/django_statsd/patches/utils.py", line 20, in wrapped
    return method(*args, **kw)
  File "/srv/login.staging.ubuntu.com/staging/code/r1190/lib/django/core/cache/backends/memcached.py", line 117, in incr
    raise ValueError("Key '%s' not found" % key)

[Martin Albisetti (beuno)]

Please make sure there are unit tests that validate that everything still works with and without memcache.
There are examples of that in ubuntuone-servers from the matvu days.