improved error message on invalid extra scripts

tests/test_extra_scripts.py

@@ -21,6 +21,15 @@ class TestExtraScripts(unittest.TestCase):
     def test_GIVEN_empty_string_THEN_returns_empty_array(self):
         self.assertEqual(get_extra_scripts(), [])
 
+    @patch("cellxgene_gateway.env.extra_scripts", new="'asdf'")
+    def test_GIVEN_bare_string_THEN_throws_Exception(self):
+        with self.assertRaises(Exception) as context:
+            self.assertEqual(get_extra_scripts(), [])
+        self.assertEqual(
+            'Error parsing GATEWAY_EXTRA_SCRIPTS, expected JSON array e.g. ["https://example.com/path/to/script.js"]',
+            str(context.exception),
+        )
+
 
 if __name__ == "__main__":
     unittest.main()