Background
----------

During integration tests, it is desirable for the application to respond
as closely as possible to the way it would in production. This improves
confidence that the application behavior acts as it should.

In Rails tests, one major mismatch between the test and production
environments is that exceptions raised during an HTTP request (e.g.
`ActiveRecord::RecordNotFound`) are re-raised within the test rather
than rescued and then converted to a 404 response.

Setting `config.action_dispatch.show_exceptions` to `true` will make the
test environment act like production, however, when an unexpected
internal server error occurs, the test will be left with a opaque 500
response rather than presenting a useful stack trace. This makes
debugging more difficult.

This leaves the developer with choosing between higher quality
integration tests or an improved debugging experience on a failure.

I propose that we can achieve both.

Solution
--------

Change the configuration option `config.action_dispatch.show_exceptions`
from a boolean to one of 3 values: `:all`, `:rescuable`, `:none`. The
values `:all` and `:none` behaves the same as the previous `true` and
`false` respectively. What was previously `true` (now `:all`) continues
to be the default for non-test environments.

The new `:rescuable` value is the new default for the test environment.
It will show exceptions in the response only for rescuable exceptions as
defined by `ActionDispatch::ExceptionWrapper.rescue_responses`. In the
event of an unexpected internal server error, the exception that caused
the error will still be raised within the test so as to provide a useful
stack trace and a good debugging experience.