tests/functional: handle URLError when fetching assets

We treat most HTTP errors as non-fatal when fetching assets,
but forgot to handle network level errors. This adds catching
of URLError so that we retry on failure, and will ultimately
trigger graceful skipping in the pre-cache task.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20250829142616.2633254-4-berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
diff --git a/tests/functional/qemu_test/asset.py b/tests/functional/qemu_test/asset.py
index 5c74adf..2dd32bf 100644
--- a/tests/functional/qemu_test/asset.py
+++ b/tests/functional/qemu_test/asset.py
@@ -15,7 +15,7 @@
 from time import sleep
 from pathlib import Path
 from shutil import copyfileobj
-from urllib.error import HTTPError
+from urllib.error import HTTPError, URLError
 
 class AssetError(Exception):
     def __init__(self, asset, msg, transient=False):
@@ -171,6 +171,14 @@ def fetch(self):
                     raise AssetError(self, "Unable to download: "
                                      "HTTP error %d" % e.code)
                 continue
+            except URLError as e:
+                # This is typically a network/service level error
+                # eg urlopen error [Errno 110] Connection timed out>
+                tmp_cache_file.unlink()
+                self.log.error("Unable to download %s: URL error %s",
+                               self.url, e.reason)
+                raise AssetError(self, "Unable to download: URL error %s" %
+                                 e.reason, transient=True)
             except Exception as e:
                 tmp_cache_file.unlink()
                 raise AssetError(self, "Unable to download: %s" % e)