Add a stopping method that stops the training process in syft.js

[Patrick Cason]

We need to have a stopping method that will terminate the current job in question. Reasons for stopping training could be any of the following:

• The user wanted to... like they clicked a "stop" button
• The plan has an error and can't execute
• The user started using their device again
• The device loses wifi
• The device loses active charging
• Or perhaps most importantly... if the model isn't really going anywhere (the error rate isn't going down)

At this point, we should stop and notify the user with some sort of message.

[Patrick Cason]

Closing in favor of #213