net/http: use DOMException.message property in error text

Previously, details about the underlying fetch error
were not visible in the net/http error text:

	net/http: fetch() failed: <object>

When using the message property, they are:

	net/http: fetch() failed: Failed to fetch
	net/http: fetch() failed: The user aborted a request.

Reference: https://developer.mozilla.org/en-US/docs/Web/API/DOMException/message.

Change-Id: Iecf7c6bac01abb164731a4d5c9af6582c250a1a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/226205


Reviewed-by: Johan Brandhorst <johan.brandhorst@gmail.com>