provider/heroku: Error on refresh if App addon is missing

[catsby]

Resource heroku_addon seems to throw an error if the App addon is deleted/removed out of band from Terraform:

• https://github.com/hashicorp/terraform/blob/master/builtin/providers/heroku/resource_heroku_addon.go#L98

The Read method (resourceHerokuAddonRead) should detect a specific "App addon no longer exists" type error, remove the app addon from state with d.SetId(""), and log the removal.

Could also implement an Exists method, like so:

• https://github.com/hashicorp/terraform/blob/master/builtin/providers/aws/resource_aws_kms_key.go#L373

[ghost]

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