vty: Return error if cmd returns CMD_WARNING while reading cfg file

Otherwise bad configurations can easily sneak in and produce unexpected
behavior.

Change-Id: Ic9c1b566ec4a459f03e6319cf369691903cf9d00
diff --git a/tests/vty/vty_test.ok b/tests/vty/vty_test.ok
index 2f76ff9..0b5ac9c 100644
--- a/tests/vty/vty_test.ok
+++ b/tests/vty/vty_test.ok
@@ -286,6 +286,10 @@
 called level3 node k
 called level1 child cmd k
 got rc=0
+reading file fail_cmd_ret_warning.cfg, expecting rc=-22
+Called: 'return-success'
+Called: 'return-warning'
+got rc=-22
 Going to test is_cmd_ambiguous()
 Going to execute 'ambiguous_nr'
 Called: 'ambiguous_nr [<0-23>]' (argc=0)