UX - Mark Duration as mandatory

**orimarko** ([Bug 62100](https://bz.apache.org/bugzilla//show_bug.cgi?id=62100&redirect=false)):
failed to execute with startup delay without Duration:

1. Check Scheduler in Thread Group

2. Enter Startup delay (in seconds) as 1

3. Keep Duration field empty

There's no error (or any record) in log although pop up refer to log.

There is no indication in UX that Duration field is mandatory.

Severity: major
OS: All