service: use parse_errno() for parsing error numbers

service: use parse_errno() for parsing error numbers

Let's always use the same logic when parsing error numbers, i.e. use
parse_errno() here too, to unify some code, and tighten the checks a
bit.

This also allows clients to pass errors as symbolic names. Probably
nothing we want to advertise too eagerly (since new daemons generating
this on old service managers won't understand), but still pretty
useful I think, in particular in scripting languages and such, where the
numeric error numbers might not be readily available.