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, 5 insertions, 1 deletions

diff --git a/actionpack/lib/action_dispatch/middleware/templates/rescues/diagnostics.html.erb b/actionpack/lib/action_dispatch/middleware/templates/rescues/diagnostics.html.erb
index bde26f46c2..999e84e4d6 100644
--- a/actionpack/lib/action_dispatch/middleware/templates/rescues/diagnostics.html.erb
+++ b/actionpack/lib/action_dispatch/middleware/templates/rescues/diagnostics.html.erb
@@ -8,7 +8,11 @@
 </header>
 
 <div id="container">
-  <h2><%= h @exception.message %></h2>
+  <h2>
+    <%= h @exception.message %>
+
+    <%= render "rescues/actions", exception: @exception, request: @request %>
+  </h2>
 
   <%= render "rescues/source", source_extracts: @source_extracts, show_source_idx: @show_source_idx, error_index: 0 %>
   <%= render "rescues/trace", traces: @traces, trace_to_show: @trace_to_show, error_index: 0 %>