Better WebAuthn and other error responses

[yaleman]

Fixes that thing where if you don't have UV you should get a slightly nicer error.

Checklist

• This pr contains no AI generated code
• cargo fmt has been run
• cargo clippy has been run
• cargo test has been run and passes
• book chapter included (if relevant)
• design document included (if relevant)

[yaleman]

@Firstyear I cleaned up the regex silliness 😄

... those cache test fails are weird, I can't tell how I touched anything in a way that'd break just them?