Validate target status for VuMark instance generation #2981
Description
When a VuMark instance generation request is made via POST /targets/<target-id>/instances for a target that is not in the success state (e.g. still processing), the mock should return a TargetStatusNotSuccessError response.
Currently, the mock generates a VuMark instance regardless of target status.
See the xfail test in vws-cli: VWS-Python/vws-cli#1968