For #6436: Adds snackbar for failed download

[sblatz]

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Pull Request checklist

• Quality: This PR builds and passes detekt/ktlint checks (A pre-push hook is recommended)
• Tests: This PR includes thorough tests or an explanation of why it does not
• Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features

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• Milestone: Make sure issues finished by this pull request are added to the milestone of the version currently in development.

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[severinrudie]

LGTM

[severinrudie]

No action needed here, but I'm concerned about BaseBrowserFragment#initializeUI. It's already 255 lines! We could pull these initializations out into helper methods, but it would mostly be a cosmetic change.

I don't know what to do here yet, but I think this is an important smell that we should be thinking about.

[codecov-io]

Codecov Report

Merging #6648 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master    #6648      +/-   ##
============================================
- Coverage     17.33%   17.32%   -0.02%     
  Complexity      390      390              
============================================
  Files           280      280              
  Lines         11206    11213       +7     
  Branches       1573     1574       +1     
============================================
  Hits           1943     1943              
- Misses         9109     9116       +7     
  Partials        154      154

Impacted Files	Coverage Δ	Complexity Δ
...a/org/mozilla/fenix/browser/BaseBrowserFragment.kt	0% <0%> (ø)	0 <0> (ø)	⬇️
...downloads/DownloadNotificationBottomSheetDialog.kt	0% <0%> (ø)	0 <0> (ø)	⬇️

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