[components] ensure dict for net.Error.headers
[components] ensure dict for net.Error.headers

This is to prevent TypeError exception like
https://screenshot.googleplex.com/3AiquHQDENKkuMP

Bug: 1138041
Change-Id: Iba06a6b57850e9b0f74da0ce1ad041e834d2c000
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2470096
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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