Validate minion retcodes

[ezh]

Original client gives warning that something wrong at least.

 > salt 'ezh.msk.ru' cmd.run 'abc'
ezh.msk.ru:
    /bin/sh: abc: command not found
ERROR: Minions returned with non-zero exit code

 > pepper ... 'ezh.msk.ru' cmd.run 'abc'
ezh.msk.ru:
    /bin/sh: abc: command not found

Fixed in #139