Report exceptions from protoc generator

[simuons]

Description

When generator fails with exception display cause stacktrace in bazel output.

Before

Exit with code 1
java.lang.RuntimeException: Exit with code 1
        at scala.sys.package$.error(package.scala:30)
        at scripts.ScalaPBWorker$.work(ScalaPBWorker.scala:56)
        at io.bazel.rulesscala.worker.Worker.persistentWorkerMain(Worker.java:92)
        at io.bazel.rulesscala.worker.Worker.workerMain(Worker.java:46)
        at scripts.ScalaPBWorker$.main(ScalaPBWorker.scala:21)
        at scripts.ScalaPBWorker.main(ScalaPBWorker.scala)

After

java.lang.RuntimeException: expected generator failure
        at test.proto.custom_generator.FailingGenerator$.run(FailingGenerator.scala:5)
        at protocbridge.frontend.PluginFrontend$.$anonfun$runWithBytes$1(PluginFrontend.scala:52)
        at scala.util.Try$.apply(Try.scala:213)
        at protocbridge.frontend.PluginFrontend$.runWithBytes(PluginFrontend.scala:52)
        at protocbridge.frontend.PluginFrontend$.runWithInputStream(PluginFrontend.scala:103)
        at protocbridge.frontend.PosixPluginFrontend$.$anonfun$prepare$1(PosixPluginFrontend.scala:33)
        at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
        at scala.concurrent.Future$.$anonfun$apply$1(Future.scala:659)
        at scala.util.Success.$anonfun$map$1(Try.scala:255)
        at scala.util.Success.map(Try.scala:213)
        at scala.concurrent.Future.$anonfun$map$1(Future.scala:292)
        at scala.concurrent.impl.Promise.liftedTree1$1(Promise.scala:33)
        at scala.concurrent.impl.Promise.$anonfun$transform$1(Promise.scala:33)
        at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:64)
        at java.base/java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1426)
        at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:290)
        at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1020)
        at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1656)
        at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1594)
        at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:177)

Exit with code 1
java.lang.RuntimeException: Exit with code 1
        at scala.sys.package$.error(package.scala:30)
        at scripts.ScalaPBWorker$.work(ScalaPBWorker.scala:56)
        at io.bazel.rulesscala.worker.Worker.persistentWorkerMain(Worker.java:92)
        at io.bazel.rulesscala.worker.Worker.workerMain(Worker.java:46)
        at scripts.ScalaPBWorker$.main(ScalaPBWorker.scala:21)
        at scripts.ScalaPBWorker.main(ScalaPBWorker.scala)