cubicweb #1337615 warn the user when remove_cube is asked whereas an instance is already running [open]

When you have an instance that is already running, and that you type besides in a cubicweb-ctl shell: "remove_cube('x')", the shell does not give back the control. You have to kill the process. It would better to do the test before executing actions such as cmd_drop_relation_type (check that there is no instances running) and if instances are running, don't execute this code and warn the user that he has to close his instances before running such commands.

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