PTFM-12428 Remove printing of stack trace for "dx run --ssh" when user's account not configured for SSH.

Summary: Additionally allow specifying DX_USER_CONF_DIR to override ~/.dnanexus_config (affects ../dxpy/scripts/dx.py: ssh_config and verify_ssh_config).

Test Plan: ./test_dxclient.py TestDXClientRun.test_dx_run_ssh_no_config

Reviewers: psung

Reviewed By: psung

Differential Revision: https://phabricator.dnanexus.com/D143