#414 Add log message

impl/src/main/java/de/fitko/fitconnect/impl/SubmissionSender.java

@@ -61,6 +61,7 @@ public class SubmissionSender implements Sender {
         try {
             return Optional.of(authService.authenticate(clientId, clientSecret, scope));
         } catch (AuthenticationException e) {
+            logger.error("client could not be authenticated", e.getMessage());
             return Optional.empty();
         }
     }