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

src/applications/conduit/controller/PhabricatorConduitAPIController.php

@@ -211,9 +211,14 @@ final class PhabricatorConduitAPIController
         ->withIsActive(true)
         ->executeOne();
       if (!$stored_key) {
+        $key_summary = id(new PhutilUTF8StringTruncator())
+          ->setMaximumBytes(64)
+          ->truncateString($raw_key);
         return array(
           'ERR-INVALID-AUTH',
-          pht('No user or device is associated with that public key.'),
+          pht(
+            'No user or device is associated with the public key "%s".',
+            $key_summary),
         );
       }