improved error message on invalid extra scripts

cellxgene_gateway/extra_scripts.py

@@ -8,6 +8,7 @@
 # the specific language governing permissions and limitations under the License.
 
 from json import loads
+from json.decoder import JSONDecodeError
 
 from cellxgene_gateway import env
 
@@ -17,4 +18,9 @@ def get_extra_scripts():
     # ['https://www.googletagmanager.com/gtag/js?id=UA-123456-2',
     #  f"{env.external_protocol}://{env.external_host}/static/js/google_ua.js"]
     # where google_ua.js is a script you add to the static/js folder prior to deployment.
+    try:
         return [] if env.extra_scripts is None else loads(env.extra_scripts)
+    except JSONDecodeError as exc:
+        raise Exception(
+            f'Error parsing GATEWAY_EXTRA_SCRIPTS, expected JSON array e.g. ["https://example.com/path/to/script.js"]'
+        ) from exc

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()