Now we have two ways to return an error to clients: appendCommandStatus() and throwing exceptions, e.g. uassertStatusOK(). It'll be great if throwing exceptions is the only way to do so.

SERVER-34256 appends error labels for certain error codes only in transaction. The information whether the user indicates running in a transaction is currently only available in service_entry_point_common.cpp, so appendCommandStatus() doesn't have enough information to append the label. We have to do that afterwards by inspecting the response.