P1 Description
The error message in the Go SDK harness around returned primaries in the self-checkpointing code (https://github.com/apache/beam/blob/ea1f292e9cf31fc8c4803b10d811f0d3ee184ae7/sdks/go/pkg/beam/core/runtime/exec/datasource.go#L375) is unclear and should be made more explicit. It should also guide the user towards making sure that the restriction behaves properly in the self-checkpointing case.
