[server] Throw ServerError on malformed verification codes

Summary:
Before, https://squadcal.org/verify/123 would cause an error that couldn't be recovered from, and the user would just see the webpage loading indefinitely.

By throwing a ServerError here, we make sure that the exception is caught here and the website still renders.

Test Plan: Make sure https://localhost/comm/verify/123 loads

Reviewers: subnub, palys-swm

Reviewed By: subnub

Subscribers: KatPo, zrebcu411, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D929