warn only if cache update fails

ubuntu-kernel-cleanup.py

@@ -61,7 +61,10 @@ def process_packages(version_number, prefix, execution_type):
     for suffix in KERNEL_SUFFIXES:
         pkg = "{}-{}{}".format(prefix, version_number, suffix)
         if execution_type == 'real':
-            CACHE.update()
+            try:
+                CACHE.update()
+            except apt.cache.FetchFailedException as err:
+                print("failed to fetch some repository: {}".format(err))
             CACHE.open()
         try:
             CACHE[pkg].is_installed  # maybe the package does not exist