[v3 onion migration] fail if v3 partial config is encountered

[conorsch]

Description

In securedrop-admin install when a user runs with v3_onion_services=True, we should:

• check for existing client auth files
• check for existing tor keypairs JSON file
• fail if one-but-not-both are found

The goal is to provide a fail-fast mechanism that prevents breaking v3 services for orgs with multiple Administrators. See discussion #4652 (comment) for context. The "validate" role seems like the best place to run this logic, since that'll catch misconfigurations even if securedrop-admin sdconfig was not run.

[redshiftzero]

If a playbook run bails after the tor keypairs JSON fail is generated but before the ths/client auth files exist, we probably shouldn't start failing subsequent attempts to run the playbook since it won't actually be due to this multiple administrator situation, it's due to a playbook failure which can happen pretty often because Tor (this happened to me while testing the other day and I thought of this ticket).

However in the scenario where the admin does have ths/client auth files but not the tor keypairs JSON file, we should fail because it means they only fetched half the secrets they need from the other admin. Lmk if you disagree @conorsch @kushaldas