'NoneType' object is not iterable_ is not a good error

[taruch]

Describe the bug
Shutting down source: dynatrace.event_driven_ansible.dt_esa_api error : 'NoneType' object is not iterable

To Reproduce
Steps to reproduce the behavior:
We received this error under two circumstances:

1. dt_api_host was incorrect - we used the FQDN as is shown in the README example without the /e/{uuid}.
   dt_api_host: "https://abc.live.dynatrace.com" or "https://abc.apps.dynatrace.com"
2. Our token had markup added to the end when it was copied, resulting in an incorrect token (unable to login)

Expected behavior
Expect an error that states you were unable to log in to the host.

[agrimmer]

I was able to reproduce the behavior: The plugin exits when the HTTP request in get_problems fails.

I did a small test on how the behavior could be fixed: https://github.com/Dynatrace/Dynatrace-EventDrivenAnsible/compare/fix/22-handle-failing-get-problems-calls?expand=1

I would be happy if anyone could take over.