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`.
Invoke mailer defaults as procs only if they are procs, do not convert
with to_proc. That an object is convertible to a proc does not mean it's
meant to be always used as a proc. Fixes#11533
Any ActionMailer class can be set to render and delier messages using
the new Rails Queue.
Some of this work was borrowed (with permission) from Nick Plante's
(zapnap) reqsue_mailer gem.
