MIL-336 -- Added Error pages to handle 404 and 500 

Changes made to the repo to handle 500 and 404

#      new file:    app/404.html 
#	modified:  server/middleware/index.js
#	new file:    server/routes/error.js
#	modified:  server/routes/index.js
#	modified:  server/server.js

Please let me know if you find any issues , I have used the legacy mockup 404 page   

http://www.crossroads.net/404.php 

for 404 page and also moved the file not found image to S3 

Below is the new S3 url for the file not found image ( just incase if anybody else need it) 

http://crossroads-media.s3.amazonaws.com/images/error.gif