Template error notification

This generates a special template for when there are template rendering *runtime* errors. This won't trigger on any case where the template would have been detected during the config check (i.e. Bosun wouldn't start). This fixes the issue where it might be very hard to reproduce the template error from the information provided in the email notification. It ensures you get a link that points to the correct instance of the alert at the time the notification was sent. 

This only impacts email notifications. So you won't see this special notification in the rule page when attempting to display a template that has a runtime error. However, if in the rule page you specify an email address, that email will show this template.

This is what the emails look like:

![image](https://cloud.githubusercontent.com/assets/1692624/5230702/69711c86-76f8-11e4-9f6d-12354fdb2654.png)

@mjibson Can you show me a code snippet on how to do that - I'm okay making that change, it just wasn't clear to me how to get that started and still have access to the context.

Also, this just seemed safer to me then rendering a template since this is kind of the last effort - but I'm okay with the change if you get me pointed in the right direction.