Add project tarball download timeout handling for airgapped environments

This MR adds guidance in case users run the Generator in airgapped environment. They should see an exception with warning that if you see timeout error please refer to docs. This should help guide them to download the tarball manually and mount it to the Generator. On case there was another error, it should be handled and passed to the Generator error handler.

Example error output:

| Vertical data: importing large projects for GPT...
Group gpt/large_projects already exists
Checking if project ff1 already exists in gpt/large_projects/not_exist1...
Tarball is remote, downloading...

GPT data generation failed:
Large project tarball download timeout.
If the machine you are running the Generator on doesn't have internet access please refer to https://gitlab.com/gitlab-org/quality/performance/-/blob/master/docs/environment_prep.md#airgapped-environments for next steps.
 Traceback:["/GitLab/performance/lib/import_project.rb:30:in `rescue in setup_tarball'", "/GitLab/performance/lib/import_project.rb:26:in `setup_tarball'", "/GitLab/performance/lib/gpt_test_data.rb:271:in `block in create_vertical_test_data'", "/GitLab/performance/lib/gpt_test_data.rb:259:in `each'", "/GitLab/performance/lib/gpt_test_data.rb:259:in `with_index'", "/GitLab/performance/lib/gpt_test_data.rb:259:in `create_vertical_test_data'", "bin/generate-gpt-data:114:in `<main>'"]