Currently, the SFE displays "An error occurred while unpausing your
account" in scenarios where it's not correct or helpful.
- Return a helpful message when a Subscriber attempts to access the
unpause form but fails to copy the entire link
- Return a helpful message when a Subscriber attempts to unpause using
an expired JWT
- Some small cleanups that make the code a little more mistake-proof.
Part of https://github.com/letsencrypt/boulder/issues/7406