Action Mailer: Declarative exception handling with `rescue_from`.

Follows the same pattern as controllers and jobs. Exceptions raised in
delivery jobs (enqueued by `#deliver_later`) are also delegated to the
mailer's rescue_from handlers, so you can handle the DeserializationError
raised by delivery jobs:

```ruby
class MyMailer < ApplicationMailer
  rescue_from ActiveJob::DeserializationError do
    …
  end
```

ActiveSupport::Rescuable polish:
* Add the `rescue_with_handler` class method so exceptions may be
  handled at the class level without requiring an instance.
* Rationalize `exception.cause` handling. If no handler matches the
  exception, fall back to the handler that matches its cause.
* Handle exceptions raised elsewhere. Pass `object: …` to execute
  the `rescue_from` handler (e.g. a method call or a block to
  instance_exec) against a different object. Defaults to `self`.

1 files changed, 4 insertions, 0 deletions

diff --git a/actionmailer/CHANGELOG.md b/actionmailer/CHANGELOG.md
index d45e74133a..3b9f503a0b 100644
--- a/actionmailer/CHANGELOG.md
+++ b/actionmailer/CHANGELOG.md
@@ -1,2 +1,6 @@
+*   Exception handling: use `rescue_from` to handle exceptions raised by
+    mailer actions, by message delivery, and by deferred delivery jobs.
+
+    *Jeremy Daer*
 
 Please check [5-0-stable](https://github.com/rails/rails/blob/5-0-stable/actionmailer/CHANGELOG.md) for previous changes.