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>
```

See merge request !5129
(cherry picked from commit 3c89a788)