"Download canceled by the user" error is wrongly displayed when you cancel saving the selection to a specific location

[SoftVision-CosminMuntean]

[Notes]:

• This issue is only reproducible if the "Always ask you where to save files" option is checked.
• The issue is not reproducible if the shot is uploaded first.

[Affected versions]:

• Nightly 59.0a1
• Beta 58.0b15

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Have a profile where the "Always ask you where to save files" option is checked in "about:preferences#general" page.

[Steps to reproduce]:

1. Open Firefox browser with the profile from prerequisites and navigate to any website.
2. Perform a selection on the page.
3. Click the download button.
4. Click the "Cancel" button from the "Save/Save as" window.
5. Observe the behavior.

[Expected result]:

• The download is canceled and no errors are displayed.

[Actual result]:

• "Error: Download canceled by the user" error is displayed.

[Additional Notes]:

• This issue is not applicable for Firefox 57 release, since the bug 1394851 was marked as "wontfix" for this version.
• Here is a screen recording with the issue:
  

[ianb]

The fix for this is to catch the specific error that happens when a user cancels a download, and ignore that error. The Browser Console will show the traceback, which should point to where the error can be caught.

[SoftVision-CosminMuntean]

This issue is no longer reproducible using latest Screenshots (32.1.0) dev version. Tested with latest Nightly (62.0a1) build on Windows 7 x64, Mac 10.13 and Arch Linux.