api: add ErrorHandler function to httputil.ReverseProxy

Change-Id: Ie706c43c9a11cf30ca0f2139a4e9eabac8c75c75
Reviewed-on: https://go-review.googlesource.com/125096
Run-TryBot: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

api/go1.11.txt

@@ -458,6 +458,7 @@ pkg net/http, type SameSite int
 pkg net/http, type Transport struct, MaxConnsPerHost int
 pkg net/http/httptrace, type ClientTrace struct, Got1xxResponse func(int, textproto.MIMEHeader) error
 pkg net/http/httptrace, type ClientTrace struct, WroteHeaderField func(string, []string)
+pkg net/http/httputil, type ReverseProxy struct, ErrorHandler func(http.ResponseWriter, *http.Request, error)
 pkg os, const ModeIrregular = 524288
 pkg os, const ModeIrregular FileMode
 pkg os, const ModeType = 2399666176