adds verbose error reporting when playback fails to find file

useful for debugging playback files.  shows what playback candidates were searched for and in which directories.

example:
irb(main):001:0> playback("nothing")
Screenshot at screenshot_0.png
RuntimeError: Playback file not found for: 'nothing' =>
  searched for =>
    * 'nothing_ios6_iphone.base64'
    * 'nothing_ios5_iphone.base64'
  in directories =>
    * '/Users/moody/git/xamarin/juke/LessPainfull/playback'
    * '/Users/moody/git/xamarin/juke/LessPainfull'
    * '/Users/moody/git/xamarin/juke/LessPainfull/features'
    * '/Users/moody/.rbenv/versions/1.9.3-p392/<snip>/lib/calabash-cucumber/resources/'