Better error message. (#768)

* Fix PrivateKeyBytes to error out and fail tests on error.

* Better error message.

caddy/https/crypto_test.go

@@ -116,5 +116,5 @@ func PrivateKeyBytes(key crypto.PrivateKey) ([]byte, error) {
 	case *ecdsa.PrivateKey:
 		return x509.MarshalECPrivateKey(key)
 	}
-	return nil, errors.New("Bad juju")
+	return nil, errors.New("Unknown private key type")
 }