changed script command to log and return an error when the submission is incomplete

This was extracted from my previous PR https://github.com/scriptcs/scriptcs/pull/803.

Even though this is now catered for in the Rosyln engine, the script command could be executed using an arbtirary engine via module overrides so I believe it has to cater for an engine returning an incomplete submission. An arbitrary engine cannot necessarily know that it is being executed in a context which does not allow for an incomplete submission and it should be able to return a `ScriptResult` with `IsCompleteSubmission` when that is the reality.