media_cache: Demote warning to debug as it may occur often.

The file playback system will now query the media cache and then
the old file functionality. Under normal conditions this will result
in the cache failing to retrieve a file causing a warning message
to get output each time a file is played back.

This change demotes this warning to a debug message.

Change-Id: Ib72246ba300b5cce32774bfb3c26634bfb708624