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

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

lib/gitlab/import_export/command_line_util.rb

@@ -29,7 +29,7 @@ module Gitlab
 
       def execute(cmd)
         output, status = Gitlab::Popen.popen(cmd)
-        @shared.error(output.to_s) unless status.zero?
+        @shared.error(Gitlab::ImportExport::Error.new(output.to_s)) unless status.zero?
         status.zero?
       end
 

lib/gitlab/import_export/saver.rb

@@ -17,7 +17,7 @@ module Gitlab
           Rails.logger.info("Saved project export #{archive_file}")
           archive_file
         else
-          @shared.error("Unable to save #{archive_file} into #{@shared.export_path}")
+          @shared.error(Gitlab::ImportExport::Error.new("Unable to save #{archive_file} into #{@shared.export_path}"))
           false
         end
       rescue => e