Improved exception handling; Return code; Test cases

9013267: Improvements re. exception handling. Defined variables for return codes and ConfigParser section name.
* Exception when evaluating regular expressions or formatting strings are caught now. These will return the unknown exit code rather than track traces.
* Added variables for global return codes in case of missing commands or failed executions. The return code for unknown/failed command executions can be set e.g. to 3 to integrate ssh-restrict in nagios-based monitoring systems.
* Some inline comments to describe the rprocess.
38c1914: Extended test cases to verify all internal checks and correct exception handling. Rewrite test cases to support python2 and python3.