targets/audio: If commit id is not found, fall back on master (take 2)

Tested in `2014-10-07_17-55-59_drinkcat_chroagh_cras-version-master-fallback-fix` (when the stars/mirrors alignment is right, the tests pass...)
Why not

```
if ! wget -O "blah" "blah" 2> "$log"; then
   cat "$log"
   echo "Error fetching CRAS"
   ...
```