proto: Implement Error for NotPrimaryMaster

When a client receives `NotPrimaryMaster` from a secondary master, the
situation is similar as when we receive an error: the other node tells,
don't connect with me, connect with someone else. Finally the peer
even closes the connection.

Due to this similarity in structure (& because it helps us later to
teach NEO/go to correctly handle `NotPrimaryMaster` with minimal
changes), we implement `Error` for `NotPrimaryMaster`.

go/neo/proto/proto-misc.go

@@ -56,6 +56,11 @@ func (e *Error) Error() string {
 }
 
 
+func (n *NotPrimaryMaster) Error() string {
+	return "not primary master"
+}
+
+
 // Set sets cluster state value to v.
 //
 // Use Set instead of direct assignment for ClusterState tracing to work.