[exec] add line break in begin failure message

haxetool/exec.js

@@ -9,7 +9,7 @@ const queue = async.queue((task, done) => {
     child_process.exec(task.command, {cwd: task.dir, maxBuffer: 1024 * 10000}, (err, stdout, stderr) => {
         if (err) {
             log.v('!%s!', err);
-            task.failure(stderr || stdout || err);
+            task.failure('\n' + (stderr || stdout || err));
         } else {
             log.v('%s', stdout);
             task.success({stdout: stdout, stderr: stderr});