Fix error messages for policy editor's yaml mode

What does this MR do and why?

This MR fixes certain error messages not being displayed in policy editor's yaml mode.

Screenshots or screen recordings

Before	After
error_handling_before	error_handling_after

How to set up and validate locally

1. Navigate to Policies -> New policy -> Scan result policy
2. Switch to yaml mode, enter the following:

   type: scan_result_policy
   name: Test
   description: ''
   enabled: true
   rules:
     - type: scan_finding
       branches: []
       scanners: []
       vulnerabilities_allowed: 0
       severity_levels: []
       vulnerability_states: []
   actions:
     - type: require_approval
       approvals_required: 2
       user_approvers_ids:
         - 4

3. Submit
4. Error alert should be shown on top
5. Remove user_approvers_ids from yaml and switch to the rule mode
6. Select Individual users from Choose approver type, select any user
7. Submit
8. Error should still be shown in the Actions section (the current behavior)

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 #395545 (closed)