find_composes prints traceback if compose directory cannot be read

[pbabinca]

    ci_list = find_composes()", 
  File \"/usr/lib/python2.6/site-packages/distill3_utils/find_latest.py\", line 58, in find_composes", 
    compose = productmd.compose.Compose(compose_path)", 
  File \"/usr/lib/python2.6/site-packages/productmd/compose.py\", line 70, in __init__", 
    for i in os.listdir(compose_path):", 
        "OSError: [Errno 13] Permission denied: '.../released/F-20/20-Beta-RC1'"

where .../released/F-20/20-Beta-RC1 is unreadable by the user.

[dmach]

This is caused by the latest code looking for metadata in legacy locations, bumping into unreadable dirs.
I'll change the code to silently ignore exceptions with errno.EACCES code.