Change flash key from :error to :alert

User session form expects a ```:alert``` key (default for Rails):

```erb
<% if flash[:alert] %>
  <div class="flash errors"><%= flash[:alert] %></div>
<% end %>
```

 but in controller you assigning error message to ```:error```:

```ruby
flash.now[:error] = t('devise.failure.invalid')
render :new
```

So if I redefine base spree layout then I will not show this error message, until customize my template to check flash ```:errors``` too.