[token-server] Make rate limit error cashtab-friendly

Summary:
The default 'message' key in 'express-rate-limit' is not automagically picked up by Cashtab, leaving rate limited users who do not know how to check the dev console confused about the error.

Change the error so it matches what is sent by other error cases, and Cashtab catches it with a toast notification.

Test Plan:
see screenshot

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D16841