bug#26252: shepherd: make error message for service not found 'smarter'

[ng0]

This is the current displayed variant:

address@hidden ~]$ sudo herd ntpd stop                                          
                                                                                
                                                              [3/3]
Password:
herd: service 'stop' could not be found
address@hidden ~]$ sudo herd stop ntpd
Service ntpd has been stopped.

**

A good solution could be to display the usage of the herd stop $foo.

~~
sudo herd ntpd stop
herd: service 'stop' could not be found.
Did you mean 'herd stop foo'?
~~

or have it display the $foo which was used.