rpc: give reason when sending non-hex raw tx

src/rpc/core_rpc_server.cpp

@@ -1250,6 +1250,7 @@ namespace cryptonote
     {
       LOG_PRINT_L0("[on_send_raw_tx]: Failed to parse tx from hexbuff: " << req.tx_as_hex);
       res.status = "Failed";
+      res.reason = "Hex decoding failed";
       return true;
     }