cron_ssacli mishandling parsing errors

in main()

```
    errors = []
    for slot in slots:
        errors += check_controller(slot)
        errors += check_array(slot)

    if len(errors) > 0:
        msg = "CRIT {} error(s): {}".format(len(errors), " - ".join(errors))
        exit = 2
    else:
        msg = "OK No errors found"
        exit = 0
```

both check_controller and check_array methods return strings or sometimes None. Using list += with strings creates an array of characters. I don't think this was the intent

This may have been intended:
```
    errors = []
    for slot in slots:
        errors.append(check_controller(slot))
        errors.append(check_array(slot))
    errors = "".join(filter(None, errors))
```