neorpc: add deprecated error codes for C#-compatibility

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:
neo-project/proposals#156
Also adding description of deprecated RPC error codes in ROADMAP.md.

Signed-off-by: Tatiana Nesterenko <tatiana@nspcc.io>

ROADMAP.md

@@ -139,3 +139,11 @@ hidden under `version` label and `server_id` contains network server ID hidden
 under `server_id` label.
 
 Removal of `serv_node_version` is scheduled for Sep-Oct 2023 (~0.105.0 release).
+
+## RPC error codes returned by old versions and C#-nodes 
+
+NeoGo retains certain deprecated error codes: `neorpc.ErrCompatGeneric`, 
+`neorpc.ErrCompatNoOpenedWallet`. They returned by nodes not compliant with the 
+neo-project/proposals#156 (NeoGo pre-0.102.0 and all known C# versions).
+
+Removal of the deprecated RPC error codes is planned once all nodes adopt the new error standard.

pkg/neorpc/errors.go

@@ -138,6 +138,21 @@ const (
 	ErrExecutionFailedCode = -608
 )
 
+var (
+	// ErrCompatGeneric is an error returned by nodes not compliant with the neo-project/proposals#156
+	// (NeoGo pre-0.102.0 and all known C# versions).
+	// It can be returned for any call and doesn't have any specific meaning.
+	//
+	// Deprecated: to be removed after all nodes adopt new error standard.
+	ErrCompatGeneric = NewErrorWithCode(-100, "RPC error")
+
+	// ErrCompatNoOpenedWallet is an error code returned by nodes not compliant with the neo-project/proposals#156
+	// (all known C# versions, NeoGo never used this code). It can be returned for wallet-related operations.
+	//
+	// Deprecated: to be removed after all nodes adopt new error standard.
+	ErrCompatNoOpenedWallet = NewErrorWithCode(-400, "No opened wallet")
+)
+
 var (
 	// ErrInvalidParams represents a generic "Invalid params" error.
 	ErrInvalidParams = NewInvalidParamsError("Invalid params")