handle failure in spark getOrCreate

[scottdraves]

enter a bad master url, like spark://1.2.3.4:7077
after a timeout of a minute, i get a stack trace and then kernel dies.

2018-06-02 15:48:10:746 -0400 [Thread-0] ERROR SparkContext - Error initializing SparkContext.
java.lang.NullPointerException
        at org.apache.spark.SparkContext.<init>(SparkContext.scala:567)
        at org.apache.spark.SparkContext$.getOrCreate(SparkContext.scala:2516)
        at org.apache.spark.sql.SparkSession$Builder$$anonfun$6.apply(SparkSession.scala:918)
        at org.apache.spark.sql.SparkSession$Builder$$anonfun$6.apply(SparkSession.scala:910)
        at scala.Option.getOrElse(Option.scala:121)
        at org.apache.spark.sql.SparkSession$Builder.getOrCreate(SparkSession.scala:910)
        at com.twosigma.beakerx.widget.SparkEngineImpl.getOrCreate(SparkEngineImpl.java:86)
        at com.twosigma.beakerx.widget.SparkEngineImpl.configure(SparkEngineImpl.java:74)
        at com.twosigma.beakerx.widget.SparkUI.initSparkContext(SparkUI.java:165)
        at com.twosigma.beakerx.widget.SparkUI.lambda$createConnectButton$0(SparkUI.java:158)
        at com.twosigma.beakerx.widget.Button.onClick(Button.java:93)
        at com.twosigma.beakerx.widget.CompiledCodeRunner.runCommEvent(CompiledCodeRunner.java:54)
        at com.twosigma.beakerx.widget.Widget.lambda$handleCommEventSync$0(Widget.java:167)
        at com.twosigma.beakerx.handler.KernelHandlerWrapper.wrapBusyIdle(KernelHandlerWrapper.java:25)
        at com.twosigma.beakerx.widget.Widget.handleCommEventSync(Widget.java:167)
        at com.twosigma.beakerx.widget.Button.handleOnClick(Button.java:88)
        at com.twosigma.beakerx.kernel.comm.Comm.handleMsg(Comm.java:297)
        at com.twosigma.beakerx.kernel.handler.CommMsgHandler.handleMsg(CommMsgHandler.java:53)
        at com.twosigma.beakerx.kernel.handler.CommMsgHandler.lambda$handle$0(CommMsgHandler.java:43)
        at com.twosigma.beakerx.handler.KernelHandlerWrapper.wrapBusyIdle(KernelHandlerWrapper.java:25)
        at com.twosigma.beakerx.kernel.handler.CommMsgHandler.handle(CommMsgHandler.java:42)
        at com.twosigma.beakerx.kernel.handler.CommMsgHandler.handle(CommMsgHandler.java:33)
        at com.twosigma.beakerx.socket.KernelSocketsZMQ.handleShell(KernelSocketsZMQ.java:190)
        at com.twosigma.beakerx.socket.KernelSocketsZMQ.run(KernelSocketsZMQ.java:165)

• there should be a spinner until it starts or errs.
• if it errs, it should return to the form (not go to connected/status).
• the form then shows the error in red, the the right of the "start" button.
• if spark API reports an error string or code, show it in red too.

i found some other errors do show in red correctly, so this issue is really about handling this specific error.

adding the spinner is not high priority, that can go in a 2nd PR.