CLI: ping/sample not properly outputting some errors, like polling timeout

[randomir]

For example:

$ dwave ping --polling-timeout 0.00001 --json
{..., "error": ""}

The error field should contain a polling timeout error description.

As a result, tests.test_cli.TestCliLive.test_ping_json_timeout_error is also failing with:

======================================================================
FAIL: test_ping_json_timeout_error (tests.test_cli.TestCliLive)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/radomir/work/dwave-cloud-client/tests/test_cli.py", line 468, in test_ping_json_timeout_error
    self.assertEqual(result.exit_code, 9)
AssertionError: 127 != 9