[#28956] No error message when saving a menu item with no title

Replaced visual clue-only (adding "invalid" class) which was missing
from template anyway by simple alter box when trying to save a menu item
with an empty title
Note: template should have:
.invalid {
	color: red;
	font-weight: bold;
}

input.invalid {
	border: 1px solid red;
}
for the bad field(s) to be displayed. Also, I removed the JS to highlight invalid field, for consistency