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, 15 insertions, 0 deletions

diff --git a/actionpack/lib/action_dispatch/middleware/templates/rescues/_actions.html.erb b/actionpack/lib/action_dispatch/middleware/templates/rescues/_actions.html.erb
new file mode 100644
index 0000000000..e181b6c539
--- /dev/null
+++ b/actionpack/lib/action_dispatch/middleware/templates/rescues/_actions.html.erb
@@ -0,0 +1,15 @@
+<% if ActiveSupport::ActionableError === exception %>
+  <% actions = ActiveSupport::ActionableError.actions(exception) %>
+
+  <% if actions.any? %>
+    <div class="actions">
+      <% actions.each do |action, _| %>
+        <%= button_to action, ActionDispatch::ActionableExceptions.endpoint, params: {
+          error: exception.class.name,
+          action: action,
+          location: request.path
+        } %>
+      <% end %>
+    </div>
+  <% end %>
+<% end %>