In the probing process, subprocess.Popen raises 'OSError : [Errno 2] No such file or directory'
and exits abruptly in the following case:
- program is not installed(e.g. run dpkg against RedHat flavors)
- program cannot be found in $PATH
- etc.

For such case, just log error message and keep on probing.

# sample output
===============================================================================
Command: dummy_command -a
===============================================================================
STDOUT:

STDERR:
[Errno 2] No such file or directory
Merge pull request #17 from quiver/error-handling-for-unknown-command

error handling for subprocess.Popen OSError