When importing, UID conflicts should be reported seprately from other errors.

Is your feature request related to a problem? Please describe.

I imported my address list and some of the requests failed. I tried to import again and I got more failures! The third import had 100% failures!

However I looked at my network requests and found the problem, a lot of HTTP 400 requests with the same error.

<?xml version="1.0" encoding="utf-8"?>
<d:error xmlns:d="DAV:" xmlns:s="http://sabredav.org/ns">
  <s:exception>Sabre\DAV\Exception\BadRequest</s:exception>
  <s:message>VCard object with uid already exists in this addressbook collection.</s:message>
</d:error>

It turns out that the "failures" were just contacts that were successfully imported the first time. It would be great for the UX if these could be reported separately from other errors. This way I can see that this is due to conflicts (and assuming that my UIDs are well-generated) know that this I am fully imported.

Describe the solution you'd like

Display a section like "400 contacts not imported because a contact with the same ID already exists (did you import this contact before?)".

Describe alternatives you've considered

No response

Additional context

No response

No files or tests are named. Start by tracing the contact import flow and how HTTP 400 responses containing the UID-conflict message are handled; done means UID conflicts appear in their own explanatory summary rather than among other import errors.