Update jwcrypto dependency to v0.5.1

This updates the jwcrypto dependency to v0.5.1, which includes some changes to the way errors are reported:

* It returns Error objects rather than strings; we should do the same here but for this deployment let's just normalize them back to strings
* It no longer distinguishes between "certificate expired" and "assertion expired" in the error message

Let's get the latest into prod and then file a follow-up bug to keep improving the error/logging situation across all these projects.

@francois r?