Prevent kernel-runner crash due to malformed service-def json files

Currently, `BaseRunner._init()` method in the `ai.backend.kernel.base` module invokes `self.service_parser.parse()` and the `parse()` method may raise `InvalidServiceDefinition` upon unexpected `IOError` and `json.JSONDecodeError` while scanning and reading the json files in the given (dir) path.

The problem is that there is no catch-guard outside (it is directly invoked in `ai.backend.kernel.__main__` as the main “setup” coroutine after instantiating the concrete class inherting `BaseRunner`), effectively **crashing** the kernel runner.

JIRA Issue: BA-2751