Raise request_canceled error instead of ambiguous_timeout for HTTP request that receive IO error

Currently we are raising an ambiguous_timeout if an http request receives an IO error and we need to stop the connection. It would seem that a request_canceled error the is more appropriate response.

We call stop() if we receive an IO error (code here). And stop() will send an ambiguous_timeout error to the handler (code here):

            if (ctx.handler) {

                ctx.handler(errc::common::ambiguous_timeout, {});

            }