Do not send ntfy messages when debugging

While debugging `xactfetch`, I do not need it to send me notifications
about failures, etc., since I am sitting at my computer.  To suppress
them, I can now set the `DEBUG_NTFY` environment variable to `0`.

xactfetch.py

@@ -288,6 +288,8 @@ class ntfyerror:
             log.exception(
                 'Swallowed exception:', exc_info=(exc_type, exc_value, tb)
             )
+            if os.environ.get('DEBUG_NTFY', '1') == '0':
+                return True
             if ss := self.page.screenshot():
                 save_screenshot(ss)
             ntfy(