OpDebug::errInfo is supposed to record "Details of any error (whether from an exception or a command returning failure)" (github reference). However, the router role SEP only sets errInfo when it catches a DBException (here), and only when it happens at that layer. It doesn't record errInfo when a command returns with an error status (i.e., here). This differs from the shard role SEP behavior.

This is complicated by the use of exceptions for control flow. For instance, the router SEP short circuits help commands here by iasserting with SkipCommandExecution. This shouldn't be recorded in errInfo, while other instances where DBExceptions are thrown should.