Check for rng-tools or catch failure if it's missing

[steveatinfincia]

Example of what happens when rng-tools isn't available:

Traceback (most recent call last):
  File "/opt/NetRNG/bin/netrngd", line 11, in <module>
    sys.exit(main())
  File "/opt/NetRNG/local/lib/python2.7/site-packages/netrng/daemon.py", line 107, in main
    client = netrng.core.Client(server_address=server_address, port=port)
  File "/opt/NetRNG/local/lib/python2.7/site-packages/netrng/core.py", line 238, in __init__ close_fds=True)
  File "/opt/NetRNG/local/lib/python2.7/site-packages/gevent/subprocess.py", line 238, in __init__ errread, errwrite)
  File "/opt/NetRNG/local/lib/python2.7/site-packages/gevent/subprocess.py", line 756, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory

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