[1.5.x] Fixed #19280 -- Raised an explicit exception for the old {% url %} syntax.

Backport of 6b8a7ce.

django/template/defaulttags.py

@@ -398,6 +398,10 @@ class URLNode(Node):
 
         view_name = self.view_name.resolve(context)
 
+        if not view_name:
+            raise TemplateSyntaxError("'url' takes requires a non-empty first"
+                " argument. The syntax changed in Django 1.5, see the docs.")
+
         # Try to look up the URL twice: once given the view name, and again
         # relative to what we guess is the "main" app. If they both fail,
         # re-raise the NoReverseMatch unless we're using the