no longer raise a rollback on merge fail

this caused the error to not be bubbled up correctly, meaning the catch in process_merge_request_job was not being triggered, and so not setting the merge back to ready

raising any exception will trigger a rollback

app/services/merge/merge_organisations_service.rb

@@ -26,7 +26,7 @@ class Merge::MergeOrganisationsService
       log_success_message
     rescue ActiveRecord::RecordInvalid => e
       Rails.logger.error("Organisation merge failed with: #{e.message}")
-      raise ActiveRecord::Rollback
+      raise
     end
   end