Fix validation in UI when updating a draft note in MR diff

What does this MR do and why?

• Resolves sub-task 4 of https://gitlab.com/gitlab-org/gitlab/-/issues/411329
• Changes the position of the error, so it's right above the invalid note instead of at the top of the page. And, it improves the error message.
• Related to !122849 (merged)

Screenshots or screen recordings

Before	After
before	after

How to set up and validate locally

1. Create a Merge Request with at least 1 file change and 1 image file added.
2. Add a validator DraftNote class since we don't have a lot of validations for comments.

# Add it to class DraftNote < ApplicationRecord in models folder

validate :test_ui_validation, on: [:create, :update]

def test_ui_validation
    if note.include? 'TEST_VALIDATION'
      self.errors.add(:base,  _('it failed validation'))
    end
end

3. Try to add comment TEST_VALIDATION in the Changes section and click on Start a review or Add to review.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #411329