Fixed #25423 -- Made error message for unknown template tag more helpful.

django/template/base.py

@@ -553,13 +553,18 @@ class Parser(object):
         if parse_until:
             raise self.error(
                 token,
-                "Invalid block tag on line %d: '%s', expected %s" % (
+                "Invalid block tag on line %d: '%s', expected %s. Did you "
+                "forget to register or load this tag?" % (
                     token.lineno,
                     command,
                     get_text_list(["'%s'" % p for p in parse_until]),
                 ),
             )
-        raise self.error(token, "Invalid block tag on line %d: '%s'" % (token.lineno, command))
+        raise self.error(
+            token,
+            "Invalid block tag on line %d: '%s'. Did you forget to register "
+            "or load this tag?" % (token.lineno, command)
+        )
 
     def unclosed_block_tag(self, parse_until):
         command, token = self.command_stack.pop()

tests/template_tests/tests.py

@@ -68,14 +68,21 @@ class TemplateTests(SimpleTestCase):
         #7876 -- Error messages should include the unexpected block name.
         """
         engine = Engine()
-
-        with self.assertRaises(TemplateSyntaxError) as e:
+        msg = (
+            "Invalid block tag on line 1: 'endblock', expected 'elif', 'else' "
+            "or 'endif'. Did you forget to register or load this tag?"
+        )
+        with self.assertRaisesMessage(TemplateSyntaxError, msg):
             engine.from_string("{% if 1 %}lala{% endblock %}{% endif %}")
 
-        self.assertEqual(
-            e.exception.args[0],
-            "Invalid block tag on line 1: 'endblock', expected 'elif', 'else' or 'endif'",
+    def test_unknown_block_tag(self):
+        engine = Engine()
+        msg = (
+            "Invalid block tag on line 1: 'foobar'. Did you forget to "
+            "register or load this tag?"
         )
+        with self.assertRaisesMessage(TemplateSyntaxError, msg):
+            engine.from_string("lala{% foobar %}")
 
     def test_compile_filter_expression_error(self):
         """