Better error message for mplot()

[garthtarr]

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.