proto: Implement Error for NotPrimaryMaster

When a client receives 'NotPrimaryMaster' from a secondary master, the
situation is similar to the situation when we receive an error: the
other node tells us, 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 'proto.NotPrimaryMaster'. Now
'NotPrimaryMaster' can be treated like an error.

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.