Disable LocalExecutor when using Client

If you submit a job using the Client, but use the LocalExecutor in your job by accident you can get weird results.
This pull requests makes the LocalExecutor throw an exception if an instance of Client was created before.
I'm not 100% happy with this implementation, since it absolutely prevents you from using LocalExecutor when you also want to create an Client instance, and perhaps there are some cases when you want to do this. What do you think?