Fix error handling in the request sender and url fetcher downloader.

That means handling the network errors by primarily looking at net_error.

Bug: 1028369
Change-Id: I8181bced25f8b56144ea336a03883d0dceea5108
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1935428
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Commit-Queue: Sorin Jianu <sorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719199}