Rename api/v1/spec.yaml -> api/v1/spec.yml
Fix inexistant filecache entries and counting their contents (#494)

The fix comes in two stages:

1. Prevent filecache entries that don't exist in the filesystem
   from passing trough cherrymodel to the client.

2. Because cherrymodel code has grown into an organic lifeform,
   also catch errors in MusicEntries in case someone tries to count
   the contents of an inexistant directory.