Error when a continuous color scale is used with a swatches legend?

[mbostock]

If you pass a continuous color scale to a swatches legend, it’s unlikely to do what you expect:

Plot.legend({
  legend: "swatches",
  color: {
    type: "linear"
  }
})

You probably want to use a quantize scale instead:

Plot.legend({
  color: {
    type: "quantize",
    scheme: "turbo"
  }
})

So, should we throw an error if you pass a continuous scale to a swatches legend?

Ref. #827