Change APT error log

tests/test_apt.py

@@ -42,7 +42,7 @@ def main():
             u.error("system out of support")
             errors += 1
         else:
-            u.error(apt_cache_err)
+            u.error("Apt error: {}".format(apt_cache_err))
             errors += 1
         apt = Apt()