Handle the case where no minion is installed

This resolves #16 if @locolupo agrees with the changes
Re-display failed responses in debug only

They have been shown to the user before that (`twisted.report_error`)
and exposing the DeferredList
fix typo introduced for red test purposes
Merge pull request #18 from yadt/fix/no_minion_error_message

Fix/no minion error message