Change confusing message informing the user that group name needs to be changed for re-import

What does this MR do and why?

This MR alters behavior of "re-import". See #383848 (closed) for details

Screenshots or screen recordings

Import_groups___GitLab

How to set up and validate locally

• Open "New group" -> Import (/groups/new)
• Use https://gitlab.com as source instance and GeK1Nis4j-SY1X4sqE5c as personal access token (this token is from separate user on GitLab instance with 0 real data available, so we do not expose any security risks here)
• Initiate group import and wait till it completes
• Observe that dropdown and input are disabled
• Click "Re-import" and observe following behavior:
  • dropdown and input are enabled
  • input is focused
  • error message that group name is already used appears
    • Note: if you've done everything too fast, you might receive HTTP 429: Rate limit on this step of validation and error message will not appear. This is known issue and it is tracked in Implement way for bulk checking of group existence (#378073 - closed)
• Enter new name
• Complete import again

MR acceptance checklist

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