Uncatched exceptions avoid clean client termination

Related to the issue that avoids polkit to be launched: after the exception has been raised, you cannot send SIGINT to the process (you have to kill -9 it).

Traceback (most recent call last):
  File "/tmp/Bitmask-linux64-2014-09-15-882d9af8/./apps/leap/bitmask/gui/mainwindow.py", line 493, in _rejected_wizard
    self.quit()
  File "/tmp/Bitmask-linux64-2014-09-15-882d9af8/./apps/leap/bitmask/gui/mainwindow.py", line 1924, in quit
    self._stop_services()
  File "/tmp/Bitmask-linux64-2014-09-15-882d9af8/./apps/leap/bitmask/gui/mainwindow.py", line 1878, in _stop_services
    self._mail_conductor.stop_mail_services()
  File "/tmp/Bitmask-linux64-2014-09-15-882d9af8/./apps/leap/bitmask/services/mail/conductor.py", line 274, in stop_mail_services
    self._firewall.stop()
  File "/tmp/Bitmask-linux64-2014-09-15-882d9af8/./apps/leap/bitmask/services/mail/emailfirewall.py", line 87, in stop
    return True if self._run(["stop"]) is 0 else False
  File "/tmp/Bitmask-linux64-2014-09-15-882d9af8/./apps/leap/bitmask/services/mail/emailfirewall.py", line 105, in _run
    pkexec = policyChecker.maybe_pkexec()
  File "/tmp/Bitmask-linux64-2014-09-15-882d9af8/./apps/leap/bitmask/util/privilege_policies.py", line 140, in maybe_pkexec
    raise NoPolkitAuthAgentAvailable()
leap.bitmask.util.privilege_policies.NoPolkitAuthAgentAvailable
2014-09-22 12:29:42 samsara leap.bitmask.gui.mainwindow[18174] DEBUG Removing service: imap
^C^C^C^C^C^C^C^C^C^C^C^C^C^C^[[B^[[B

(from redmine: created on 2014-09-22, closed on 2014-12-18, relates #6123 (closed), relates #6150 (closed))