Reverse the check for http response code.

There are many codes other than '404' which indicate that the media
file is not present or otherwise unusable. Instead of checking for
a particular failure mode, check for explicit success.

I ran into this on a server that had directory listing disabled, and
returned a 403 (access denied) error on non-existant files.
Merge pull request #308 from kepstin/presentation-200

Reverse the check for http response code.