Accept response validation error handler

[nampas]

Here's a stab at the feature outlined here: #454

This PR adds a new validateResponses.onError option. There's no strong contract with the function, but express-openapi-validator will pass the err object, as well as the response data. Usage might look something like this

const handler = (err, data) => {
   console.log('OpenAPI response validation error', err);
};

OpenApiValidator.middleware({
   apiSpec: './spec/openapi.json',
   validateRequests: true,
   validateResponses: { onError: handler }
});

[cdimascio]

can you also add a test, where you re-throw the error and/or create a new error. looks like should all work out, but having a test will be good.

[cdimascio]

thanks for this @nampas

[cdimascio]

one more note, please update the validateResponses section of the README to describe this new option

[cdimascio]

@nampas thanks for the feature