Add reassignment error to Import::SourceUser

What does this MR do and why?

Add reassignment error to Import::SourceUser

This MR adds an error column to import source user records. This can be used to track why a particular reassignment failed, which can then be displayed in the front-end as per the below issue.

Issue: #473975 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Enable the :importer_user_mapping feature flag.
2. Generate test data:

   user = User.first
   
   flight = Group.find_by(path: 'flightjs')
   
   # Template source user
   template = Import::SourceUser.new(
     placeholder_user: user,
     reassign_to_user: user,
     status: 1,
     source_hostname: "example.com",
     import_type: "github",
     reassigned_by_user: user
   )
   
   # Create 10 source users on the toolbox group.
   1.upto(10) do |num|
     new = template.dup
     new.namespace = flightjs
     new.source_username = "flight#{num}",
     new.source_name = "Flight User #{num}",
     new.source_user_identifier = "flu#{num}",
     new.save!
   end
   
   # Give some of them error messages
   flight.import_source_users.sample(5).each do |isu|
     isu.update(reassignment_error: SecureRandom.hex)
   end

3. Run the query in GraphiQL and check the response:

   {
     group(fullPath: "flightjs") {
       importSourceUsers {
         nodes {
           id
           reassignmentError
         }
       }
     }
   }