Merge branch 'fix/import-export-prod' into 'master'

Fixed a couple of errors on import/export spotted in production

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/18828 and https://gitlab.com/gitlab-org/gitlab-ce/issues/18829

A couple of tiny bugs spotted after deploying in prod

See merge request !4782

app/services/projects/import_export/export_service.rb

@@ -50,7 +50,7 @@ module Projects
       end
       def notify_error
-        notification_service.project_not_exported(@project, @current_user, @shared.errors.join(', '))
+        notification_service.project_not_exported(@project, @current_user, @shared.errors)
       end
     end
   end

lib/gitlab/import_export.rb

@@ -21,7 +21,7 @@ module Gitlab
     end
     def config_file
-      'lib/gitlab/import_export/import_export.yml'
+      Rails.root.join('lib/gitlab/import_export/import_export.yml')
     end
     def version_filename