[token-server] Improve recaptcha handling

Summary: Useful to see failed recaptchas in the logs. Also, no need to check validity of recaptcha if we do not get a recaptcha token.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16855