t_CheckExitStatus should take an optional message

[nkk1]

functional tests writers will want t_CheckExitStatus to take an optional message instead of
exit_status=$?
t_Log 'message'
t_CheckExitStatus $exit_status

[tigalch]

Feel free to supply an pull request with your suggestions regarding code change

[nkk1]

Here is the pull request #14 awaiting couple of days :)