When another instance of the simulator is running, it crashes in an ugly way

[gabeabrams]

Make a prettier crash than this:

events.js:183
      throw er; // Unhandled 'error' event
      ^

Error: listen EADDRINUSE :::8088
    at Object._errnoException (util.js:992:11)
    at _exceptionWithHostPort (util.js:1014:20)
    at Server.setupListenHandle [as _listen2] (net.js:1355:14)
    at listenInCluster (net.js:1396:12)
    at Server.listen (net.js:1480:7)
    at module.exports (/Users/gabeabrams/code/students-at-risk-widget/node_modules/caccl-canvas-partial-simulator/index.js:122:10)
    at Object.<anonymous> (/Users/gabeabrams/code/students-at-risk-widget/node_modules/caccl/canvas/startPartialSimulation.js:76:1)
    at Module._compile (module.js:652:30)
    at Object.Module._extensions..js (module.js:663:10)
    at Module.load (module.js:565:32)