Vagrantfile: Stop processing in case of errors

If something in the provisioning of the vagrant virtual machine goes
wrong the developer might not notice (the creation log is very long),
but the virtual machine will be more or less subtly broken. Instead,
halting on errros makes problems immediately apparent.

Link: https://github.com/ValveSoftware/Proton/pull/4343

Vagrantfile

@@ -65,6 +65,7 @@ Vagrant.configure(2) do |config|
     debian10.vm.synced_folder ".", "/home/vagrant/proton", id: "proton", type: "rsync", rsync__exclude: ["vagrant_share"]
 
     debian10.vm.provision "shell", privileged: "true", inline: <<-SHELL
+      set -e
       #install docker and steam-runtime dependencies
       dpkg --add-architecture i386
       apt-get update