Email target should only get notified on transition to error state

[AnthonySteele]

Email target should only get notified on transition into error state, and not on transition into Ok state, as the pagerduty email trigger can't tell them apart and makes false alarms.

@tomhaigh says in #3:

We could create an SNS topic for each type of notification target - i.e. an email one (containing all email subscriptions) and a url one (with all url subscriptions) for each alerting group. That would mean we could restrict what types of notification go to each type of target without additional configuration.

The alternative is to stop supporting email alerts. But they are useful for trialing so rather keep them.

[AnthonySteele]

This now works for services made in cloudformation (i.e. all but DynamoDb tables, SQS Queues)

[tomhaigh]

Should we close this now as the plan is to use CloudFormation for everything?