Return JSON on error (game/%%/move)

[benjaminpjones]

For some reason, html is being returned and it confuses JSON.stringify(), so the error shown isn't very useful. I think this has something to do with the fact that we are calling next() on error (a fix I had to do to deal with express not knowing how to handle unhandled promise exceptions!)

Screenshot (should say something like "you can't place a stone on an existing stone"):