An error is not displayed if the upload/download process has failed due to internet disconnection

[SoftVision-CiprianMuresan]

[Affected versions]:

• Firefox 54.0 and up

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Steps to reproduce]:

1. Start the Firefox browser with and navigate to "https://send.dev.mozaws.net/" page.
2. Upload a medium to large file (>100 MB).
3. While the file is uploading, disconnect the computer from the internet.
4. Wait 1-2 minutes and observe the behavior.

[Expected result]:

• After a while an error view appears informing the user that he no longer has internet.

[Actual results]:

• The upload process does not continue but the page is stuck at whatever percent it managed to reach.

[Notes]:

• The issue is also reproducible if the internet is disconnected while downloading a file.
• The issue is also reproducible on https://send.stage.mozaws.net/.
• Here is a screen recording with the issue:
  

[SoftVision-CosminMuntean]

I have retested this issue on dev and stage servers and the behavior is a little bit changed. If you lose the internet connection while uploading/downloading a file, you are automatically redirected to the Send upload page.
With this behavior the user is not informed that the upload/download failed due to the internet connection.

Here is a screen recording with the issue:

[ehuggett]

ref: #551