Allow log_disk_space script to exit early

What does this MR do and why?

Re: Force job to fail early if the runner's startin... (#498142 - closed)

With sufficient disk space:

• minimum required disk space set as 2048MB / 2GB
• example job: https://gitlab.com/gitlab-org/gitlab/-/jobs/8059064690 ✅

With insufficient disk space:

• I bumped up the minimum threshold to 2TB from 2048MB to force this failure.
• example job: https://gitlab.com/gitlab-org/gitlab/-/jobs/8058994015 ❌ failed

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