Possible improvement on errorResponder

Hi @kimmobrunfeldt , first of all thank you very much for your work.
I noticed that the error response was not homogeneous, when it comes to errors from Puppeteer.
To trigger a "managed" error:
`{ output: "screenshot", screenshot: { type: "ABC", ... }`
This will return a json with error code, message, error list etc..

To trigger an "unmanaged" error:
`{ output: "screenshot", screenshot: { type: "png", fullPage: true, clip: { 0, 0, 32, 32 } }`
This will return only a status 500 and message of "Internal server error"

I've changed it for my deployment, and maybe the idea might be implemented on your side, if you consider it.
The chance will make the ErrorResponder send back always a json with the same schema even when there's a "unhandled" error (like in the example.

Here's my code for "errorResponder" in src\middleware\error-responder.js
```
return function errorResponder(err, req, res, next) {
const status = err.status ? err.status : 480;
var httpMessage = http.STATUS_CODES[status];
httpMessage = httpMessage ? httpMessage : err.message;

const errBody = _.has(err, 'errors')
? err.errors
: [{fields: [], location: "body", messages:[err.message]}];

res.status(status);
res.send({ status, statusText: httpMessage, errors: errBody });
};
```

Thanks, cheers
Andy