Show error(s) that prevented project from being created, or the repository from being migrated.

Problem

Currently when some errors appeared during GitHub import which marks import as Failed the only status is shown on UI without an actual error.

Proposal

Add an error message to {{host}}/import/github/status and {{host}}/import/github/realtime_changes endpoints (into the import_error field) to be able to show error(s) that prevented the project from being created or the repository from being migrated.

Technical details

The error message is already stored in import_failures table so the error message has to be added to app/serializers/project_import_entity.rb