httpError: write error string as body in 502 response

[omribahumi]

When CONNECT fails, other than getting a 502 response, there's no indication of the failure reason.

This pull request will:

1. Print the dial failure reason to the Go log output
2. Return the error in the 502 response body

I couldn't test this with curl as it suppresses the body for 5xx responses for some reason. Not sure if this is because this is a 502 in response to the CONNECT call.

Before:

% echo $'CONNECT nodns.boo HTTP/1.1\r\n' | nc localhost 8080
HTTP/1.1 502 Bad Gateway

After:

% echo $'CONNECT nodns.boo HTTP/1.1\r\n' | nc localhost 8080
HTTP/1.1 502 Bad Gateway
Content-Type: text/plain
Content-Length: 40

dial tcp: lookup nodns.boo: no such host

[nkns165]

@elazarl It looks like a good change, why don't you merge it? I'd love to have this warning log.

[elazarl]

@nkns165 I'm worried it'd log too much for some people, maybe we need a better tunable logs.

[nkns165]

Thank you so much...!