Store the request object in proxy errors

[rowanmanning]

This would allow someone to differentiate between client errors and
target server errors in their proxy error handling, by inspecting the
error request property.

I'm not sure if this is how you'd prefer to do this, open to alternatives.
The problem I'm trying to solve is in a proxy service I maintain – I'd
like to differentiate between client and server errors in our logging.

This is a potential solution for #1017. E.g:

proxy.on('error', (error, request, response, url) => {
    // Will log either "target" or "forward"
    console.log(`Error came from ${error.request.proxyRequestType}`);
});

[rowanmanning]

Tests are failing in Node.js 0.10.x/0.12.x, but it's unrelated to my changes. See #1167.

[jcrugzz]

@rowanmanning I hesitate to add such a heavy object with circular references to the error (because this could cause issues if someone tries to log the error object potentially) but I'm not opposed to attaching metadata so that we can detect the request type and any other relevant information. Is there anything besides type you would want to add?

[rowanmanning]

Hi @jcrugzz, your hesitance makes perfect sense. I'd be happy to have type metadata, and can't think of anything else off the top of my head. If you don't have time to do the work yourself then I'd also be happy to have a go myself, just might need pointing in the right direction :)