If async validator throws an error, support using that error's message instead of mongoose's message templating

[vkarpov15]

Re: issues like #6772. Right now the best workaround looks like this:

schema.path('foo').validate({
  validator,
  message: props => props.reason.message
});

I'm not sure what this API would look like though, looking for suggestions.

[vkarpov15]

Fixed in 5.6.0, see #4913