operations: Implement structured errors for UserSquash

This MR converts UserSquash to return structured errors. Before, the RPC call returned successfully in some error conditions, which is making us blind for errors and thus causes us to schedule unneeded replication jobs.

Please note: before we merge this MR we need to make changes in Rails to proprely handle those errors. So I'm putting this up for review first such that we agree on behaviour, but then I'll first have to land downstream changes before we merge this.

Part of #4052 (closed)