Fix log statements in import/export

Fixes - as seen in the logs:

Import/Export error raised on /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/import_export/file_importer.rb:19:in `rescue in import': undefined method `message' for #<String:0x007fc45e977e10>

@rymai reaaally quick one for review again - related to logging issues so I didn't add a changelog as it's transparent for the user...

Reassigned to @rymai

Status changed to merged

mentioned in commit 3c89a788

mentioned in issue #19568 (closed)

Picked into 8-9-stable, will be in 8.9.6.

Removed ~149423 label

mentioned in commit 46e8efc2