Merge branch 'fix/gitlab-import-project-file-fix' into 'master'

Fix temp file being deleted after the request while importing a GitLab project

Fixes https://gitlab.com/gitlab-com/infrastructure/issues/151

In production, the temporary uploaded file is getting deleted straight after the request so the Sidekiq worker is unable to find it in `/tmp`

Also, improved erroring/logging of this situation.

See merge request !4894