Add error if emulator is already running

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Instructions

Thrown errors when the emulator is already running are not very explicit by nature. For instance if I attempt to run the kitty-items tests with the emulator already running, the error I see is thrown: undefined. This is can be very confusing as evinced by #89 .

(Optional): Suggest A Solution

Add some sort of exception handling related to emulator already running, throw an error that is more explicit.

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Closing as this is superseded by #112 and #113