net/http/httputil: add Error handler in ReverseProxy

[juliens]

I would love to be able to customize the RoundTrip error handling.
We should be able to change the default behavior that always writes StatusBadGateway.

For example:

statusCode := http.StatusInternalServerError
if e, ok := err.(net.Error); ok {
	if e.Timeout() {
		statusCode = http.StatusGatewayTimeout
	} else {
		statusCode = http.StatusBadGateway
	}
} else if err == io.EOF {
	statusCode = http.StatusBadGateway
}
w.WriteHeader(statusCode)
w.Write([]byte(http.StatusText(statusCode)))

[bradfitz]

Dup of #21255?

There's a pending fix at https://go-review.googlesource.com/c/go/+/54030

[gopherbot]

Change https://golang.org/cl/77410 mentions this issue: http.ReverseProxy: add an error handler

[odeke-em]

And CL 77410 has been merged to become commit 5ea2360. Please reopen if that fix doesn't suffice.

Thank you for the proposal/feature-request @juliens and @bradfitz for keeping tabs!

[gopherbot]

Change https://golang.org/cl/78495 mentions this issue: http.ReverseProxy: add an error handler

[tmthrgd]

@odeke-em should this be reopened since 24df1d0 reverted 5ea2360?

[odeke-em]

Sure, thanks for the vigilance @tmthrgd!

[rsc]

/cc @tombergan?

[bradfitz]

I like the ErrorHandler approach more than overloading ModifyResponse.

Let's go with a cleaned-up CL 78495. I'll send the minimal edits required.