Added additional Transmitter tests, and some associated fixes

This includes some tweaks such that errors are raised for certain
commands
DOC: added a note that coverage script is python-coverage on Debian systems
Merge branch 'transmitter-testcase' of https://github.com/kwirk/fail2ban

* 'transmitter-testcase' of https://github.com/kwirk/fail2ban:
  Added additional Transmitter tests, and some associated fixes