frequent login issues

several users have been complaining recently about «Il y a un problème avec votre compte. Merci de contacter l'administrateur du site.»

At least some of them can be correlated with an `Unexpected error: Invalid salt` in the logs, when POSTing to `/users/login`.

```
Traceback (most recent call last):
File "/var/www/.build/venv/lib/python3.4/site-packages/pyramid/tweens.py", line 22, in excview_tween
response = handler(request)
File "/var/www/.build/venv/lib/python3.4/site-packages/pyramid_tm/__init__.py", line 109, in tm_tween
reraise(*exc_info)
File "/var/www/.build/venv/lib/python3.4/site-packages/pyramid_tm/compat.py", line 15, in reraise
raise value
File "/var/www/.build/venv/lib/python3.4/site-packages/pyramid_tm/__init__.py", line 88, in tm_tween
response = handler(request)
File "/var/www/.build/venv/lib/python3.4/site-packages/pyramid/router.py", line 158, in handle_request
view_name
File "/var/www/.build/venv/lib/python3.4/site-packages/pyramid/view.py", line 547, in _call_view
response = view_callable(context, request)
File "/var/www/.build/venv/lib/python3.4/site-packages/pyramid/config/views.py", line 182, in __call__
return view(context, request)
File "/var/www/.build/venv/lib/python3.4/site-packages/pyramid/viewderivers.py", line 393, in attr_view
return view(context, request)
File "/var/www/.build/venv/lib/python3.4/site-packages/pyramid/viewderivers.py", line 371, in predicate_wrapper
return view(context, request)
File "/var/www/.build/venv/lib/python3.4/site-packages/pyramid/viewderivers.py", line 442, in rendered_view
result = view(context, request)
File "/var/www/.build/venv/lib/python3.4/site-packages/pyramid/viewderivers.py", line 147, in _requestonly_view
response = view(request)
File "/var/www/.build/venv/lib/python3.4/site-packages/cornice/service.py", line 485, in wrapper
response = view_()
File "/var/www/c2corg_api/views/user.py", line 433, in post
token = try_login(user, password, request) if user else None
File "/var/www/c2corg_api/security/roles.py", line 68, in try_login
if user.email_validated and user.validate_password(password):
File "/var/www/c2corg_api/models/user.py", line 142, in validate_password
return PasswordUtil.is_password_valid(plain_password, self._password)
File "/var/www/c2corg_api/models/user.py", line 51, in is_password_valid
return bcrypt.hashpw(plain, encrypted) == encrypted
File "/var/www/.build/venv/lib/python3.4/site-packages/bcrypt/__init__.py", line 86, in hashpw
raise ValueError("Invalid salt")
ValueError: Invalid salt
```