R1:eb80a5ede1fb

Make the Conduit auth error for an unrecognized public key a little more useful

Summary: Ref T13168. This is just a small quality-of-life fix: we can disclose which public key we're talking about because public keys are public.

Test Plan:
- Hit public key error (through my own bumbling / not reading or following instructions). Specifically, I haven't associated the key with a device in Almanac.
- Before: vague error.
- After: more specific error with enough key material that I could grep for it.