Do not specifiy to user why login failed

Do not specify to user why the login failed because it makes our
services insecure. Instead tell users why login failed through a generic
flash message.This is easier to implement anyway since there are no
model level validations for session.

Create a flash message and partial for account creation failures. We
want the flash message on top so that the error for account creation is
congruent with the error message for account login. Account creation on
the other hand will in addition specify why failure has occurred through
display model level validation errors. These errors will be rendered
through a partial.