Remove tmp image if the download operation fails

If the download operation fails for any reason, subsequent attempts
result in a `File exists @ rb_sysopen - /tmp/ubuntu-jammy.img.tmp
(Errno::EEXIST)` error. In case of failure, the temporary image must be
removed.

Fixes #911

rhizome/host/lib/vm_setup.rb

@@ -451,6 +451,9 @@ def download_boot_image(boot_image, custom_url: nil)
         else
           r "curl -f -L10 -o #{temp_path.shellescape} #{download.shellescape}"
         end
+      rescue CommandFail
+        rm_if_exists(temp_path)
+        raise
       end
 
       # Images are presumed to be atomically renamed into the path,

rhizome/host/spec/vm_setup_spec.rb

@@ -63,6 +63,19 @@ def key_wrapping_secrets
       vs.download_boot_image("github-ubuntu-2204", custom_url: "https://images.blob.core.windows.net/images/ubuntu2204.vhd?sp=r&st=2023-09-05T22:44:05Z&se=2023-10-07T06:44:05")
     end
 
+    it "cleans up tmp image if the downloading fails" do
+      expect(File).to receive(:exist?).with("/var/storage/images/ubuntu-jammy.raw").and_return(false)
+      expect(File).to receive(:open) do |path, *_args|
+        expect(path).to eq("/tmp/ubuntu-jammy.img.tmp")
+      end.and_yield
+      expect(FileUtils).to receive(:mkdir_p).with("/var/storage/images/")
+      expect(Arch).to receive(:render).and_return("amd64").at_least(:once)
+      expect(vs).to receive(:r).with("curl -f -L10 -o /tmp/ubuntu-jammy.img.tmp https://cloud-images.ubuntu.com/releases/jammy/release-20231010/ubuntu-22.04-server-cloudimg-amd64.img").and_raise(CommandFail.new("Couldn't download image", "", "404 Not Found"))
+      expect(FileUtils).to receive(:rm_r).with("/tmp/ubuntu-jammy.img.tmp")
+
+      expect { vs.download_boot_image("ubuntu-jammy") }.to raise_error CommandFail
+    end
+
     it "can use an image that's already downloaded" do
       expect(File).to receive(:exist?).with("/var/storage/images/almalinux-9.1.raw").and_return(true)
       vs.download_boot_image("almalinux-9.1")