closes when destination folder has been cleared

[ghost]

Step to reproduce on Debian:

• run the updater so that the game can work
• remove everything in the download folder, except the unvanquished folder
• run the updater

What happens:

A popup saying Failed to start Unvanquished process. shows, and the updater ends.

What would be expected:

Updater should at least ask to repair (aka: reinstall).

[ghost]

Of course, clicking on the "settings" wheel allows to force the updater to download, but I think this should be done automatically. Clicking on that wheel is "not so simple" and normal users could think they need more extreme measures.

[slipher]

Checking the full installation integrity is not feasible, but I guess we could do a minimal check that the engine binary exists

[ghost]

It could be possible to check if the daemon executable fails to start, or returns too quickly. It could also be possible to make the updater place a marker that the client would remove if successfully started. If marker is still there when updater is run anew, then show an emergency window or something. Just some random ideas.