net/http: add response body to 413 and 400 errors

net/http: add response body to 413 and 400 errors

The existing serve() method returns a zero-length response body when
it encounters an error, which results in a blank page and no visible
error in browsers.

This change sends a response body explaining the error for display in browsers.

Fixes #12745

Change-Id: I9dc3b95ad88cb92c18ced51f6b52bd3b2c1b974c
Reviewed-on: https://go-review.googlesource.com/15018
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>