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)

This should be a const.