Fixing code review
Adding exit 1, in case of error
Fixing code review
Parallel upload/download for dx-upload/download utilities

Test Plan: updated unit tests.

Reviewers: psung

Differential Revision: https://phabricator.dnanexus.com/D71
Merging with other bash-helpers branch
Merge branch 'bash-helpers-parallel' of github.com:dnanexus/dx-toolkit into bash-helpers-parallel