Catch installation errors and display them.

[almet]

This catches the errors that can happen during installation, and during some other phases, and display them to the end user in the interface.

Fixes #193

[almet]

On MacOS, the tests are currently failing. I managed to reproduce on our mac mini. For a reason I still need to understand, the following command hangs indefinitely.

usr/local/bin/docker image list --format "{{.ID}}" dangerzone.rocks/dangerzone

And as a result the GUI continues showing that it's "installing" the image.

[almet]

Tests are now passing (without doing any changes). I'm not sure if it was due to an error with the runners? 🤔
I re-triggered them and they seem to be green each time now.

@apyrgio I believe this is ready for review.

[apyrgio]

Tested the PR on a Windows environment. Aside from a stray exception, everything else seems to work nicely. Nice job!

[apyrgio]

Not much else to comment on, I think it's good to merge.

[almet]

Tests were intermittently failing on MacOS and Windows, due to the way we tested threads, which was a bit brittle. I changed the approach to flatten threads during the tests (replacing .start() with .run()) to ensure the logic is tested properly.

If that's okay for you @apyrgio, and that tests are green on all targets, I'll wait until #748 is merged, rebase and we should be good.