better error handling

[maxd-nordic]

this happens when the device is already registered to another account:

(.env) max@max-MS-7C91:~/work/repos/ncs/nrf/samples/debug/memfault$ nrf_cloud_onboard --api-key $API_KEY_PROD && rm onboard.csv
INFO Devices to be onboarded: 1
INFO Onboarding API call result: 202 - Accepted
INFO Fetching results for bulkOpsRequestId: dfghdfh
INFO Waiting 5s...
INFO Onboarding status: FAILED
ERROR Failure during onboarding, downloading error summary...
ERROR Unhandled error response format
Traceback (most recent call last):
File "/home/max/work/repos/ncs/.env/bin/nrf_cloud_onboard", line 8, in
sys.exit(run())
^^^^^
File "/home/max/work/repos/ncs/.env/lib/python3.12/site-packages/nrfcloud_utils/nrf_cloud_onboard.py", line 563, in run
main(sys.argv[1:])
File "/home/max/work/repos/ncs/.env/lib/python3.12/site-packages/nrfcloud_utils/nrf_cloud_onboard.py", line 552, in main
res = do_onboarding(args.api_key, args.csv, args.res, args.chk)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/max/work/repos/ncs/.env/lib/python3.12/site-packages/nrfcloud_utils/nrf_cloud_onboard.py", line 437, in do_onboarding
err_count, err_json = parse_err_msg(error_results.text)
^^^^^^^^^^^^^^^^^^^
TypeError: cannot unpack non-iterable NoneType object