While our server no longer uses these codes (-100, -400) they still can come
from C# servers and while we consider them deprecated we better at least have
some definition of them until C# implements our proposal:
https://github.com/neo-project/proposals/pull/156
Also adding description of deprecated RPC error codes in ROADMAP.md.
Signed-off-by: Tatiana Nesterenko <tatiana@nspcc.io>