fixup: restore rpcerror proto message

hivemind/proto/p2pd.proto

@@ -174,6 +174,10 @@ message PSResponse {
   repeated bytes peerIDs = 2;
 }
 
+message RPCError {
+  optional string message = 1;
+}
+
 message CallUnaryRequest {
   required bytes peer = 1;
   required string proto = 2;