Fix frontend validation in 'new poll' form

Part of #975

What does this MR do?

Two error message in the front-end validations were not shown:

• if the end date is before the start date
• if an option is added but does not have a text yet

How confident are you it won't break things if deployed?

Very sure

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

1. Checkout branch locally
2. Login as userbot
3. Go to region -> polls -> new poll
4. Select an end date that is before the start date -> the message should be visible (screenshot 1)
5. Fill out the options and click the spinner to add a new option -> the message should be visible (screenshot 2)

Screenshots (if applicable)

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• add a short text that can be used in the release notes
• Once your MR has been merged, you are responsible to create a testing issue in Beta Testing Repo:
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...). how this change can be tested.
  • Be aware, that also non technical people should understand.

Release notes text

• Fix im Abstimmungstool:
  • Warnung, wenn das Ende Datum vor dem Anfangsdatum liegt.
  • Berücksichtigung wenn eine neue Option hinzugefügt wird, die aber noch keinen text hat. (A short text that will appear in the release notes and describes the change for non-technical people. Not always necessary, e.g. not for refactoring.)