API for not imported records

Problem

In case when Direct transfer completed, but not all records were imported, the user need is to identify the records that failed, and understand why they failed to be imported. This not available yet.

Proposal

Create new failures API endpoint so that it returns enough information for user to understand what records failed to import and why.

Frontend can use this API endpoint to show information in UI, issue.

Any API caller (with permissions) can get info from this endpoint as well.

Save, for each failed record:

• What kind of record it is (e.g. merge request, issue, etc.)
• The ID of the record on the source (e.g. the merge request ID, issue ID, etc.) - we don't export this, see comment
• A "human readable" identifier of the record, if present (e.g. the merge request title, the issue title, etc.)
• What kind of error happened and caused the record to not be imported
• The correlation ID for debugging purposes
• (Nice to have) URL of the record - should be possible to recreate if a record has iid. Not all URLs can be recreated ( e.g. protected branches).

Documentation

Document new API endpoint.