cmd/go/internal/web: include snippets of plain-text server responses in error detail

cmd/go/internal/web: include snippets of plain-text server responses in error detail

For the server response to be displayed, the response must be served
as type text/plain with charset us-ascii or utf-8, and must consist of
only graphic characters and whitespace.

We truncate the server response at the first blank line or after 8
lines or a fixed number of characters, and tab-indent (if multiple
lines) to ensure that the response is offset from ordinary go command
output.

Fixes #30748

Change-Id: I0bc1d734737e456e3251aee2252463b6355e8c97
Reviewed-on: https://go-review.googlesource.com/c/go/+/189783
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>