Policy Executor: handle errors, part 2

Policy Executor: handle errors, part 2
(fighting between IntelliJ and Checkstyle best practices)

- non-2xx responses are processed using web client exceptions
- handle unknown host exception
- upgraded spotbugs (checkstyle and related mvn plugin)
- fixed some small spotbugs due to upgrade
- added commented instructions in docker compose to enable debugging
- added some environment variables for policy executor configuration
- extract out Sleeper in stub service to achieve 100% coverage
- added cause to Policy Executor exceptions where applicable
- ignored (new) spotbug rule about catch NPE because of issue in 3pp
- ignored (new) spotbug rule about \n in string due to multiline string block

Issue-ID: CPS-2412
Change-Id: I6835a73320c436cbeea12cc7a06f15899eec7bf1
Signed-off-by: ToineSiebelink <toine.siebelink@est.tech>