Handle `run_on_cluster` error that occurs when sim$results dataframes don't match

Currently, if different simulation replicates return sim$results dataframes with different structures, the following uncaught error will occur:

```R
Error in rbind(deparse.level, ...) : 
  numbers of columns of arguments do not match
```

This error should be caught/handled with an appropriate error message. Note that this may be an issue when running locally as well.