Report native lib compilation failures

Fix scala-native#692

sbt-scala-native/src/main/scala/scala/scalanative/sbtplugin/ScalaNativePluginInternal.scala

@@ -240,7 +240,7 @@ object ScalaNativePluginInternal {
             logger.running(compilec)
             val result = Process(compilec, cwd) ! logger
             if (result != 0) {
-              println("Failed to compile native library runtime code.")
+              sys.error("Failed to compile native library runtime code.")
             }
         }
       }