If setting a system-variable provided by a plug-in failed, no OK or
error was sent in some cases, hanging the client. We now send an error
in the case from the ticket (integer-argument out of range in STRICT
mode). We also provide a semi-generic fallback message for possible
future cases like this where an error is signalled, but no message is
sent to the client. The error/warning handling is unified so it's the
same again for variables provided by plugins and those in the server
proper.