MINOR: server: enhance error precision when applying server state

MINOR: server: enhance error precision when applying server state

server health checks and agent parameters are written the same way as
others to be able to enahcne code reuse: basically we make use of
parsing and assignment at the same place. It makes it difficult for
error handling to know whether srv object was modified partially or not.
The problem was already present with SRV resolution though.

I was a bit puzzled about the approach to take to be honest, and I did
not wanted to go into a full refactor, so I assumed it was ok to simply
notify whether the line was failed or partially applied.

Signed-off-by: William Dauchy <wdauchy@gmail.com>