Report each failed client request to the error logger once

A failed request could reach the client's error logger twice: once from
the step that failed, without a request when that was the TLS setup or
the proxy CONNECT, and again from send_() with the request. A content
receiver or progress callback that canceled was reported twice too, a
CONNECT answered with 407 only without a request, and a stream that
failed to open not at all unless TLS failed.

Report a failure in one place instead. The outermost send() reports the
request's error once, with the request the caller made, also when the
failure happened on a redirect (a redirect client no longer gets the
error logger) and when an answer carries an error, such as a proxy's
407. open_stream() reports a stream that fails to open the same way.
Only an upload body compressed before send() still reports where it
fails.

docs-src/pages/ja/cookbook/c19-client-logger.md

@@ -35,7 +35,7 @@ cli.set_error_logger([](const httplib::Error &err, const httplib::Request *req)
 });
 ```
 
-第2引数の`req`はヌルポインタのこともあります。TLSハンドシェイクやCA証明書の読み込み、プロキシのトンネル確立など、接続の準備中にエラーが起きた場合です。使う前に必ずヌルチェックしてください。
+失敗したリクエストは、送ったリクエストと一緒に1回だけ報告されます。リダイレクト先で失敗した場合も同じです。`open_stream()`や`stream::Get()`でストリームを開くのに失敗した場合も同じように報告され、ボディを読む途中のエラーは`read_error()`で返ります。`req`がヌルポインタになるのは、どのリクエストにも属さないエラーをサーバーのエラーロガーが受け取るときだけです。
 
 ## 両方を組み合わせる