Fix promise output when falling back to SR python

On python3 slapos.core for python2 software release, the output was bytes:

```
2021-11-05 17:28:39 slapos[12654] INFO Error with promises for the following partitions:
2021-11-05 17:28:39 slapos[12654] INFO   slappart7[balancer]: b'Promise \'check-apachedex-result.py\' failed with output: ERROR \'"/srv/slapgrid/slappart3/srv/runner/software/cc0326f0dcb093f56c01291c300c8481/bin/check-apachedex-result" --apachedex_path "/srv/slapgrid/slappart3/srv/runner/instance/slappart7/srv/monitor/private/apachedex" --status_file /srv/slapgrid/slappart3/srv/runner/instance/slappart7/srv/monitor/private/apachedex.report.json --threshold "70"\' run with failure, output: \'Score too low: 0% - Threshold: 70.0%\\n\''
```

See merge request nexedi/slapos.core!344