Fail early if starting Scala.js sub-process failed

Author: alexarchambault

modules/build/src/main/scala/scala/build/internal/Runner.scala

@@ -19,7 +19,7 @@ import scala.build.errors.{
   TooManyFrameworksFoundByBridgeError
 }
 import scala.build.testrunner.{AsmTestRunner, TestRunner}
-import scala.util.Properties
+import scala.util.{Failure, Properties, Success}
 
 object Runner {
 
@@ -282,6 +282,12 @@ object Runner {
       val config    = RunConfig().withLogger(logger.scalaJsLogger)
       val processJs = envJs.start(inputs, config)
 
+      processJs.future.value.foreach {
+        case Failure(t) =>
+          throw new Exception(t)
+        case Success(_) =>
+      }
+
       val processField =
         processJs.getClass.getDeclaredField("org$scalajs$jsenv$ExternalJSRun$$process")
       processField.setAccessible(true)