Add API allowing plugins to declare the current action invalid

[delucis]

Currently we support returning INVALID_MOVE from moves to bail out of a state update. There are circumstances where a plugin might also want to flag that an action is invalid. For example, a plugin that validates state or — as came up in #957 (review) — in the events plugin if there is an error processing the event queue.

This would probably look something like the current noClient plugin method. A plugin could declare an isInvalid method that could return any truthy value to invalidate the current action.

const plugin = {
  name: 'validator',
  isInvalid: ({ G }) => {
    if (typeof G.value !== number) {
      return { error: 'G.value must be a number' };
    }
  },
};

Once we have something like #723, plugin errors could be added to the possible error types and the return value of isInvalid returned to the client for debugging.