Report bad SSH configuration to the user

[smondet]

One example:
http://www.undeadly.org/cgi?action=article&sid=20070925181947

a proxy without 2> /dev/null generates "Killed by signal 1" messages that mess up with Ketrew's parsing, without much information:

        * 2015-06-03-23h14m35s603ms-UTC: Tried_to_eval_condition
            Host: SSH failed parsing
            log:ketrew_2015-06-03-23h14m32s952ms-UTC_396223738ketrew_2015-06-03-23h14m32s952ms-UTC_3962237381Killed
 [Log → "Return value not an integer"]
        ...