bug#73879: [shepherd] 'herd start x' error should be clearer when its been disabled

[Maxim Cournoyer]

Hi,

Currently, attempting to start service x when it was disable gives
something like:

--8<---------------cut here---------------start------------->8---
herd: error: impossible to start service x
--8<---------------cut here---------------end--------------->8---

while the actual root cause is that the service has been disabled.

The error should probably just print something like:

--8<---------------cut here---------------start------------->8---
herd: error: service x is disabled; run 'herd enable x' to enable it
--8<---------------cut here---------------end--------------->8---

-- 
Thanks,
Maxim