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/Makefile.am b/tests/Makefile.am
index e0993b1..60213ed 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -293,6 +293,7 @@
 	     vty/fail_not_de-indented.cfg \
 	     vty/fail_tabs_and_spaces.cfg \
 	     vty/fail_too_much_indent.cfg \
+	     vty/fail_cmd_ret_warning.cfg \
 	     vty/ok.cfg \
 	     vty/ok_empty_parent.cfg \
 	     vty/ok_ignore_blank.cfg \