Major - P3 errmsg is populated when performing normal inserts:
mongos> db.runCommand({ insert: '', documents: [{ x: 1 }]})
{ "ok" : 0, "code" : 10094, "errmsg" : "invalid ns: test.", "n" : 0 }
However, errmsg is empty when run against the system.indexes collection
mongos> db.runCommand({ insert: "system.indexes", documents: [ { _id: ObjectId('528a423ea29a612fec1561d7'), ns: "test", key: { x: 1.0 }, name: "x" } ], ordered: true })
{ "ok" : 0, "code" : 26, "errmsg" : "", "n" : 0 }