Fix gallery

I replaced exceptions.
And, I added action for timer in return for them.

def SlideShow(self, time=10):
"""
:doc: gallery method
         
This is the action to be set to timer for slideshow.
            
`time`
      This is a number in second, defaults to 10. advance to the next unlocked image
       when this time is elapsed if slideshow is enable.
"""
I also change diff argument to be default to True.