More explicit phoenix template rendering

[paulcsmith]

Right now view rendering uses Phoenix.View.render_existing which is ok except that it can make it easy to think you're sending an email with both formats, but in reality you had a typo in a template name and you only send a text or html version. Instead I think this should be more explicit so you get more immediate and helpful feedback.

Right now you do: render("template_name") which will render a text or html template if it exists

Instead I think it should be render("template_name.html") and render("template_name.text") if you have just one type of template to send.

If you want to send text and html then you would do render(:template_name) and it will render both and raise if either one is missing.