Consider conffile prompts to be errors when there's nothing else to do

Unless there are also packages that can be upgraded, processing won't get
beyond "exit if there is nothing to do" to report a failure result because
of conffile prompts.

Check for conffile prompts when determining success/failure at the earlier
"nothing to do" stage.

Fixes mvo5#336.

unattended-upgrade

@@ -2371,7 +2371,7 @@ def run(options,             # type: Options
 
         pkgs_kept_back = cache.find_kept_packages(options.dry_run)
         return UnattendedUpgradesResult(
-            kernel_pkgs_remove_success,
+            kernel_pkgs_remove_success and not pkg_conffile_prompt,
             _("No packages found that can be upgraded unattended and no "
               "pending auto-removals"),
             pkgs_removed=kernel_pkgs_removed,