Further improved exit code standardization

(branch is merged already, but comments anyway...)

```python 
_("Date entered is invalid. Date should be in YYYY-MM-DD format (example: ") + 
   strftime("%Y-%m-%d", localtime()) + " )"
```
Might be troublesome for translators, since it leaves them having
to use a trail "( blah blah ". 

Not that we don't have other translated strings with similar constructs...

maybe use str.format?

```python
                    msg = _("Date entered is invalid. Date should be in YYYY-MM-DD format (example: {dateexample})")
                    dateexample = strftime("%Y-%m-%d", localtime())
                    system_exit(os.EX_DATAERR,
                                msg.format(dateexample=dateexample))
```