What you are asking kubectl to do is copy the file catalina.2017-05-02.log to the current context, but the current context is a directory. The error is stating that you can not copy a file to have the name of a directory.
Try giving the copied version of the file a name:
kubectl cp my-namepace/my-pod:/opt/tomcat/logs/catalina.2017-05-02.log -c my-container ./catalina.2017-05-02.log.