No output generated if REST API responds with incomplete JSON

[haxtibal]

For certain execution errors at the API side, the REST API may respond with a JSON like

{"Invoke-IcingaCheckService" :{"exitcode":{}, "checkresult": "", "perfdata": {}}}

call_api_check.exe currently handles the case by returning an empty string and exit code 3 = unknown.

Exit code = unknown is correct (we can't know what the real result would have been), but we should additionally print a message that can be displayed as summary. Maybe something like "Incomplete API response for Invoke-IcingaCheckService".