improve the error message returned when peer verification fails

crypto.go

@@ -93,7 +93,11 @@ func (i *Identity) ConfigForPeer(remote peer.ID) (*tls.Config, <-chan ic.PubKey)
 			return err
 		}
 		if remote != "" && !remote.MatchesPublicKey(pubKey) {
-			return errors.New("peer IDs don't match")
+			var peerID peer.ID = "(not determined)"
+			if p, err := peer.IDFromPublicKey(pubKey); err == nil {
+				peerID = p
+			}
+			return fmt.Errorf("peer IDs don't match. Expected %s, got %s", remote, peerID)
 		}
 		keyCh <- pubKey
 		return nil

transport_test.go

@@ -184,7 +184,8 @@ var _ = Describe("Transport", func() {
 		}()
 		// dial, but expect the wrong peer ID
 		_, err = clientTransport.SecureOutbound(context.Background(), clientInsecureConn, thirdPartyID)
-		Expect(err).To(MatchError("peer IDs don't match"))
+		Expect(err).To(HaveOccurred())
+		Expect(err.Error()).To(ContainSubstring("peer IDs don't match"))
 		Eventually(done).Should(BeClosed())
 	})