`ScatterRegions` not returning which region(s) are failed to scatter

[D3Hunter]

Bug Report

What did you do?

ScatterRegions not returning which region(s) are failed to scatter. br/lightning now will prefer ScatterRegions to do region scatter for performance reason, but if some regions failed to scatter for reasons like region %d is not fully replicated, it will also return success and only give a percentage of success regions, but doesn't tell client which failed. currently br/lightning will take this as success, and will move on, and cause region/data skew.

br/lightning have retry on scatter, but need PD returns which regions have failed

https://github.com/tikv/pd/blob/2bbeb9c97141c0f16d99d407b97c2d2366adfd38/pkg/mcs/scheduling/server/grpc_service.go#L219-L232

What did you expect to see?

What did you see instead?

What version of PD are you using (pd-server -V)?