title: Invalid password format or unknown hashing algorithm
body: <p>One of the users on my site recently managed to trigger this traceback while attempting to login. In Django Admin, his password reads <code>Invalid password format or unknown hashing algorithm.</code></p>
  
  <p>I have no idea what might have caused this. So far this has been an isolated case and I and other users have managed to successfully sign up and login to the site.</p>
  
  <p><strong>Traceback</strong></p>
  
  <pre><code>Traceback (most recent call last):
  
   File "/home/gituser/.virtualenvs/bbox/lib/python2.7/site-packages/django/core/handlers/base.py", line 111, in get_response
     response = callback(request, *callback_args, **callback_kwargs)
  
   File "/var/git/bbox/userprofile/views.py", line 67, in login_view
     if form.is_valid():
  
   File "/home/gituser/.virtualenvs/bbox/lib/python2.7/site-packages/django/forms/forms.py", line 124, in is_valid
     return self.is_bound and not bool(self.errors)
  
   File "/home/gituser/.virtualenvs/bbox/lib/python2.7/site-packages/django/forms/forms.py", line 115, in _get_errors
     self.full_clean()
  
   File "/home/gituser/.virtualenvs/bbox/lib/python2.7/site-packages/django/forms/forms.py", line 271, in full_clean
     self._clean_form()
  
   File "/home/gituser/.virtualenvs/bbox/lib/python2.7/site-packages/django/forms/forms.py", line 299, in _clean_form
     self.cleaned_data = self.clean()
  
   File "/var/git/bbox/userprofile/forms.py", line 83, in clean
     self.user_cache = authenticate(username=username, password=password)
  
   File "/home/gituser/.virtualenvs/bbox/lib/python2.7/site-packages/django/contrib/auth/__init__.py", line 45, in authenticate
     user = backend.authenticate(**credentials)
  
   File "/home/gituser/.virtualenvs/bbox/lib/python2.7/site-packages/django/contrib/auth/backends.py", line 15, in authenticate
     if user.check_password(password):
  
   File "/home/gituser/.virtualenvs/bbox/lib/python2.7/site-packages/django/contrib/auth/models.py", line 304, in check_password
     return check_password(raw_password, self.password, setter)
  
   File "/home/gituser/.virtualenvs/bbox/lib/python2.7/site-packages/django/contrib/auth/hashers.py", line 42, in check_password
     hasher = get_hasher(algorithm)
  
   File "/home/gituser/.virtualenvs/bbox/lib/python2.7/site-packages/django/contrib/auth/hashers.py", line 115, in get_hasher
     "setting?" % algorithm)
  
  ValueError: Unknown password hashing algorithm ''. Did you specify it in the PASSWORD_HASHERS setting?
  </code></pre>