Added a new callback to promise-with-error conversion

Author: ldaniels528

core/src/main/scala/io/scalajs/util/PromiseHelper.scala

@@ -83,6 +83,19 @@ object PromiseHelper {
     task
   }
 
+  /**
+    * Converts a JavaScript-style callback to a Scala.js promise
+    * @param f the given callback function
+    * @return a Scala.js promise
+    */
+  @inline
+  def promiseWithError0[Z](f: js.Function1[Z, Any] => Unit): Promise[Unit] = {
+    val task = Promise[Unit]()
+    f((err: Z) =>
+      if (err == null || js.isUndefined(err)) task.success({}) else task.failure(wrapJavaScriptException(err)))
+    task
+  }
+
   /**
     * Converts a JavaScript-style callback to a Scala.js promise
     * @param f the given callback function