When the mongod server responds to the client with something like the following (mongo shell in this example):

MongoDB Enterprise mongos> show dbs
2019-04-08T17:54:08.081+0000 E QUERY    [js] Error: listDatabases failed:{
    "ok" : 0,
    "errmsg" : "cannot add session into the cache",
    "code" : 261,
    "codeName" : "TooManyLogicalSessions",

a corresponding error is not logged to the mongodb.log for the mongoS, making troubleshooting more difficult if the error from the application side is not immediately available. This seems like a low-hanging-fruit improvement we can make to facilitate troubleshooting.