Inform user which resources were not migrated

Problem

Currently we show errors that occurred during migration, but we don't inform users which resources (issues, MRs etc.) were not migrated. This info is important to users, so that they can decide if they are fine with those resources not being migrated or they need to repeat migration or fix it in some other way.

For a problem of not showing when a user wasn't mapped correctly during the migration we have a separate issue: Feedback when a user wasn't mapped correctly du... (#394736).

Proposed solution

Show general end state of migration process: Co... (#394727 - closed) is about showing the general end result of migration (Completed, Partially completed, Failed) with a badge.

With this issue we would show to the user:

• for Partially completed state
  • exactly which entities were not imported (as in which MRs exactly, for example), if possible provide a direct link to that entity on source instance
  • errors related to entities not getting imported, attachments not migrated, etc. so user (and ourselves) have more information to troubleshoot/debug. Reason could also be timeout.
• for Failed state - show error(s) that prevented group(s) from being created. Reason could also be timeout.

Now we have it like this:

and what we want is that, when user click on See failures button, all relations that failed to import (MR1, MR2 etc) are shown in a list. List items also show the errors that caused the relation to fail importing.

The UI could look like the equivalent page for the GitHub importer: the Details button links to a page that displays a table, with each line showing a failed relation.

Titles of table columns: Type, Title, Error, Correlation ID.