Added check to 404 video poster

Verifies there is a poster image for the 404 video before printing it.

Fixes #3559 

@aaronschachter 
Kinda picky, but does it make sense to only output poster= if the variable is set? Also should the value be in quotes? Same with the Video src?  I don't know how all this new HTML5 stuff works / what is best practice :older_man: