Error handling in processes created for UserParameter command execution is inconsistent.

For go agent on all platforms and for c agent on windows item becomes not supported in a lot of cases. However on unix platforms execution failures are ignored and the error message is returned to the item as item value.

This affects all kind of external command execution.
https://git.zabbix.com/projects/ZBX/repos/zabbix/browse/src/libs/zbxexec/execute.c

Suggest making this behavior consistent across different operating systems, as well as across old and new agent.