Fix download component behavior when network connection is cut.

[dero]

Summary

Fixes #126

Currently when network connection is lost while a video is being downloaded, the downloader component gets stuck and doesn't either explicitly pause the download or wait for the connection to come back up and resume automatically.

To address this, this PR makes sure that when there is a connection related error encountered by the DownloadManager, the current downloads are paused automatically and the UI of the downloader component changes from the "downloading" state to "paused". At the same time a console warning is generated. User can then manually resume the affected downloads once connection is restored.

While this is likely not a perfect solution, it may be good enough or may serve as a first step towards a more robust solution. Please let me know your thoughts, @derekherman and / or @beaufortfrancois.

Screencast of the updated behavior

web-dev-download-interrupted-warning.mp4

[beaufortfrancois]

This PR definitely improves the current state of downloading in kino. Thank you @dero!

I was surprised to notice background fetch was not used there. If background fetch is there, kino should use it, otherwise it should fallback to "regular" fetch. Is there a feature request for this yet? @rkochman FYI

[dero]

Background fetch was originally intended to be implemented during phase 2 (this phase), see #25, but I'm not sure if that still is the plan.

[rkochman]

Not including Background Fetch was an oversight on my part. We should try to get it in Phase 2.

[derekherman]

This works to address the original issue and can be merged. Additionally, we have now prioritized adding background fetch to this phase—we can do that in a new PR.

[felipebochehin87]

Bug found: #176

[felipebochehin87]

Bug found: #183