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...)
Well, my method has the advantage of displaying the progress as the file is being downloaded (wget outputs status+errors in stderr), without adding a huge amount of complexity (piping status in new fds, etc.). The tarball is not _that_ huge (402k), but can still take a bit of time to fetch...