Clean up how missing end tags are handled in template parsing.

The null token was an artificial placeholder that wasn't making clear what
the problem was. Throwing an exception is bolder.

tools/lib/template_parser.py

@@ -211,16 +211,8 @@ def validate(fn=None, text=None, check_indent=True):
         elif kind == 'django_end':
             state.matcher(token)
 
-    null_token = Token(
-        kind=None,
-        s='(NO TAG)',
-        tag='NO TAG',
-        line=0,
-        col=0,
-    )
-
     if state.depth != 0:
-        state.matcher(null_token)
+        raise TemplateParserException('Missing end tag')
 
 def is_special_html_tag(s, tag):
     # type: (str, str) -> bool

tools/tests/test_template_parser.py

@@ -85,6 +85,13 @@ class ParserTest(unittest.TestCase):
         '''
         self._assert_validate_error('Bad indentation.', text=my_html, check_indent=True)
 
+    def test_validate_state_depth(self):
+        # type: () -> None
+        my_html = '''
+            <b>
+        '''
+        self._assert_validate_error('Missing end tag', text=my_html)
+
     def test_validate_incomplete_handlebars_tag_1(self):
         # type: () -> None
         my_html = '''