Add hook for deploy failure

If an error is raised during a deploy, the task `deploy:failed` will be
triggered.  Custom tasks can hook into this using `after`:

    after 'deploy:failed', :send_for_help do
      #
    end

I've also taken the opportunity to provide a marginally more useful
error message before triggering the task.

By default, this 'deploy:failed' will only be triggered when running
`cap <stage> deploy` - to trigger after individual tasks use `set
:deploying, true`

This closes #708 and replaces
https://github.com/capistrano/capistrano/pull/720