don't crash if bcache doesn't exists

if bcache doesnt't exists or loaded collectd-bcache crashes at start with message

bcache is not loaded.
Traceback (most recent call last):
  File "./collectd-bcache", line 139, in <module>
    main()
  File "./collectd-bcache", line 117, in main
    if len(uuids) == 0:
TypeError: object of type 'NoneType' has no len()
call sleep if bcache doesn't exist
print debug message as comment

as described here http://collectd.org/documentation/manpages/collectd-exec.5.shtml
each line beginning with a # (hash mark) is ignored.