[net] add headers to errors and logs
[net] add headers to errors and logs

When a request fails and response headers are available, log them and include in
the raised Error.

Bug: 819453
Change-Id: I3118bd4dc4f8fd0b1712a3efbb937979847246b5
Reviewed-on: https://chromium-review.googlesource.com/974645
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py