[mythtv-cmyth] Check if channel icons exist before trying to load them

Before downloading the channel icons, we should make sure that they exist.
The fix checks if they exist in the ChannelIcons storage group that was introduced by 0.25. So this will break icons for older backends.

Probably it's better to use [QUERY_FILE_EXISTS](http://www.mythtv.org/wiki/QUERY_FILE_EXISTS_%28Myth_Protocol%29) instead

The issue was described in the forum: http://forum.xbmc.org/showthread.php?tid=110694&pid=1365524#pid1365524

```
Mar 14 07:14:27 mythserv00 mythlogserver: mythbackend[15509]: I ProcessRequest mainserver.cpp:1510 (HandleAnnounce) MainServer::HandleAnnounce FileTransfer
Mar 14 07:14:27 mythserv00 mythlogserver: mythbackend[15509]: I ProcessRequest mainserver.cpp:1512 (HandleAnnounce) adding: buildroot as a remote file transfer
Mar 14 07:14:27 mythserv00 mythlogserver: mythbackend[15509]: E ProcessRequest fileringbuffer.cpp:285 (OpenFile) FileRingBuf(/home/administrator/.mythtv/channels/bbc_cbeebies_asia.jpg): OpenFile(): Could not open.
```