crptonote_core: do not error out sending unparsable extra field

extra is arbitrary, and the user may well want to send custom data

src/cryptonote_core/cryptonote_tx_utils.cpp

@@ -294,8 +294,8 @@ namespace cryptonote
     }
     else
     {
-      LOG_ERROR("Failed to parse tx extra");
-      return false;
+      MWARNING("Failed to parse tx extra");
+      tx_extra_fields.clear();
     }
 
     struct input_generation_context_data