[processing] Fix file download alg stuck in infinit loop when network errors occur

[nirvn]

Description

This PR fixes a bug whereas the file downloader algorithm gets stuck in an infinite loop when network errors occur. Long story short here: do not throw when inside an event loop and expect things will terminate properly :)

[Gustry]

Thanks for this fix.
I just got it yesterday as well.

[qgis-bot]

The backport to queued_ltr_backports failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-queued_ltr_backports queued_ltr_backports
# Navigate to the new working tree
cd .worktrees/backport-queued_ltr_backports
# Create a new branch
git switch --create backport-44094-to-queued_ltr_backports
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 4f6f4be425e77446cda063b116ba49124a5486d0
# Push it to GitHub
git push --set-upstream origin backport-44094-to-queued_ltr_backports
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-queued_ltr_backports

Then, create a pull request where the base branch is queued_ltr_backports and the compare/head branch is backport-44094-to-queued_ltr_backports.