thumbor: Fix exceptions if LOCAL_UPLOADS_DIR is not set.

This is the typical configuration in /etc/zulip/settings.py for a site
using S3, since there's no obligation to set the setting to None
directly, and we're not importing the main `settings.py` (and thus not
getting DEFAULT_SETTINGS applied).

zthumbor/thumbor.conf

@@ -19,7 +19,10 @@ config_file.read("/etc/zulip/zulip.conf")
 # Whether this instance of Zulip is running in a production environment.
 PRODUCTION = config_file.has_option('machine', 'deploy_type')
 if PRODUCTION:
+    try:
         from zproject.prod_settings import LOCAL_UPLOADS_DIR
+    except ImportError:
+        LOCAL_UPLOADS_DIR = None
 else:
     from zproject.dev_settings import LOCAL_UPLOADS_DIR