coverage: Bring zerver.lib.narrow.py to 100% coverage.

tools/test-backend

@@ -73,7 +73,6 @@ not_yet_fully_covered = {
     'zerver/lib/events.py',
     'zerver/lib/i18n.py',
     'zerver/lib/message.py',
-    'zerver/lib/narrow.py',
     'zerver/lib/notifications.py',
     'zerver/lib/push_notifications.py',
     'zerver/lib/request.py',

zerver/tests/test_narrow.py

@@ -21,6 +21,7 @@ from zerver.lib.message import (
 from zerver.lib.narrow import (
     build_narrow_filter,
 )
+from zerver.lib.request import JsonableError
 from zerver.lib.str_utils import force_bytes
 from zerver.lib.sqlalchemy_utils import get_sqlalchemy_connection
 from zerver.lib.test_helpers import (
@@ -342,6 +343,11 @@ class BuildNarrowFilterTest(TestCase):
             for e in reject_events:
                 self.assertFalse(narrow_filter(e))
 
+    def test_build_narrow_filter_invalid(self):
+        # type: () -> None
+        with self.assertRaises(JsonableError):
+            build_narrow_filter(["invalid_operator", "operand"])
+
 class IncludeHistoryTest(ZulipTestCase):
     def test_ok_to_include_history(self):
         # type: () -> None