Fix the error response constructed in ClientRegistrationEndpoint.validateCertificate(HttpServletRequest).

The second parametger given to the OBBDCRProcessor.errorResponse method should
be an error code, not a format string for the error description.

Author: TakahikoKawasaki

src/main/java/com/authlete/jaxrs/server/api/ClientRegistrationEndpoint.java

@@ -203,7 +203,8 @@ private static void validateCertificate(HttpServletRequest request)
         catch (GeneralSecurityException e)
         {
             throw OBBDCRProcessor.errorResponse(Status.UNAUTHORIZED,
-                    "Client certificate validation failed", e.getMessage());
+                    "invalid_client",
+                    String.format("Client certificate validation failed: %s", e.getMessage()));
         }
     }
 }