Introduce Actionable Errors

Actionable errors let's you dispatch actions from Rails' error pages. This
can help you save time if you have a clear action for the resolution of
common development errors.

The de-facto example are pending migrations. Every time pending migrations
are found, a middleware raises an error. With actionable errors, you can
run the migrations right from the error page. Other examples include Rails
plugins that need to run a rake task to setup themselves. They can now
raise actionable errors to run the setup straight from the error pages.

Here is how to define an actionable error:

```ruby
class PendingMigrationError < MigrationError #:nodoc:
  include ActiveSupport::ActionableError

  action "Run pending migrations" do
    ActiveRecord::Tasks::DatabaseTasks.migrate
  end
end
```

To make an error actionable, include the `ActiveSupport::ActionableError`
module and invoke the `action` class macro to define the action. An action
needs a name and a procedure to execute. The name is shown as the name of a
button on the error pages. Once clicked, it will invoke the given
procedure.

1 files changed, 3 insertions, 1 deletions

diff --git a/actionpack/lib/action_dispatch/middleware/debug_exceptions.rb b/actionpack/lib/action_dispatch/middleware/debug_exceptions.rb
index 59113e13f4..a679f9bf01 100644
--- a/actionpack/lib/action_dispatch/middleware/debug_exceptions.rb
+++ b/actionpack/lib/action_dispatch/middleware/debug_exceptions.rb
@@ -4,6 +4,8 @@ require "action_dispatch/http/request"
 require "action_dispatch/middleware/exception_wrapper"
 require "action_dispatch/routing/inspector"
 
+require "active_support/actionable_error"
+
 require "action_view"
 require "action_view/base"
 
@@ -19,7 +21,7 @@ module ActionDispatch
     end
 
     def initialize(app, routes_app = nil, response_format = :default, interceptors = self.class.interceptors)
-      @app             = app
+      @app             = ActionableExceptions.new(app)
       @routes_app      = routes_app
       @response_format = response_format
       @interceptors    = interceptors