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, 1 insertions, 0 deletions

diff --git a/activesupport/lib/active_support.rb b/activesupport/lib/active_support.rb
index 5589c71281..40a9181493 100644
--- a/activesupport/lib/active_support.rb
+++ b/activesupport/lib/active_support.rb
@@ -33,6 +33,7 @@ require "active_support/core_ext/date_and_time/compatibility"
 module ActiveSupport
   extend ActiveSupport::Autoload
 
+  autoload :ActionableError
   autoload :Concern
   autoload :CurrentAttributes
   autoload :Dependencies