The issue presented in #26246 showed a deeper underlying problem. When
we fell back to the exception handler for an exceptions cause, we were
calling that handler with the outer raised exception. This breaks the
calling code's expectations, especially if the exception has methods on
it behond those from `StandardError`.
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`.
