Feature request

I propose having a nice explainable warning that informs the user one cannot plot MDS with insufficient number of topics.

Motivation

The issue was opened many times: #1914, #1010, #725, #378. This means people get confused by the original error. Having an integrated error issued after a check on the number of topics in the topic model would be beneficial.

Your contribution

I looked into the code, but honestly didn't know where such a warning would be most appropriate and how to format it.