wal-e: Insufficient temporary space leads to unclear errors on backup-push

When there is insufficient temporary space (/tmp) during a backup-push IOError 32 (broken pipe) is raised. This issue may not be limited to backup-push.

Root cause

Insufficient /tmp space

Mitigation

1. Allocate more /tmp space. (May not be possible in some environments)
2. Perform backup-push with --pool-size=1 to resolve higher space requirements from default of multiple workers multiplying the /tmp load.

Fixes

1. Document the above. --pool-size does not seem to be documented anywhere, I just happened to come across it while diagnosing this issue. I hope that this issue will help others encountering this problem.
2. Check for available temporary space for each worker before starting and error out gracefully.
3. Allow the user to specify a temporary path to override the default.
4. (Minor) If possible, determine if insufficient space (IOError 12) is the root cause for the broken pipe and error out gracefully.