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.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: yelirekim

Maniphest Tasks: T13168

Differential Revision: https://secure.phabricator.com/D19516