Project export: Improve error message logging for ImportExport::Saver#compress_and_save failures

In https://log.gprd.gitlab.net/goto/ee1ebbc0-7e14-11ec-a649-b7cbb8e4f62e, I see errors such as:

Since some of these projects may have large external files (e.g. build artifacts), I suspect the tar command may be failing due to space issues, but it's not clear to me that's the case.

https://gitlab.com/gitlab-org/gitlab/blob/c2da24f85e4a56a100c59ad565ccd7aaee09e6a7/lib/gitlab/import_export/command_line_util.rb#L80-90 should be saving stdout/stderr, but in https://log.gprd.gitlab.net/goto/3d043c60-7e1a-11ec-a649-b7cbb8e4f62e, all I see is:

Zendesk tickets:

1. https://gitlab.zendesk.com/agent/tickets/260712