PTFM-13536: confusing error by 'dx cp'

Summary: Refactoring of the 'dx cp' command, adding another test case to the test suite.

Test Plan: Additional test case added to the TestDXCp test suite.

Reviewers: klai

Differential Revision: https://phabricator.dnanexus.com/D169
Merge branch 'PTFM-13536-dx-cp-confusing-error' of github.com:dnanexus/dx-toolkit into PTFM-13536-dx-cp-confusing-error