fixd bug: not stdout when cmd error and not stderr

fixed bug like this:
```
[root@test ~]# cat test.sh
exit 123
[root@test ~]# sh test.sh
[root@test ~]# echo $?
123
```
```
[root@salt salt]# salt  'test' cmd.run_stderr './test.sh'
test:
```

after fixed:

[root@salt salt]# salt  'test' cmd.run_stderr './test.sh'
test:
    False
```