Better error message for mplot()

When running the `mplot()` function it's common to forget to add the original fitted model object as the first argument, in which case you get this uninformative error:

```
> mplot(mm_vis)
Error in input_list[[j]] : subscript out of bounds
```

Need to add in a better error that tells the user that the first argument (`mf = ...`) needs to be a fitted model object.