This is continuation of SERVER-29428:

When pointing a 3.4 mongos at a featureCompatibilityVersion=3.6 cluster, the mongos spins, logging:
remote host has incompatible wire version: IncompatibleServerVersion: Server
min and max wire version are incompatible (6,6) with client min wire version
(5,5)
Instead, the processes should quickly exit with a non-zero exit code and a useful error message. The error message should display a "MustUpgrade" error code and should link to the documentation for the 3.4=>3.6 upgrade process.

This is not possible because mongos uses a DBConnectionPool, which uses a MongoURI that does not return a status code, but sets an error string. MongoURI::connect should return a StatusWith<DBClientConnection> to determine the reason for connection failure, which can be checked in Refresher::_refreshUntilMatches in replica_set_monitor.cpp

DBClientBase* DBConnectionPool::get(const MongoURI& uri, double socketTimeout) {
     std::unique_ptr<DBClientBase> c(_get(uri.toString(), socketTimeout));
     if (c) {
         onHandedOut(c.get());
         return c.release();
     }

     string errmsg;
     c = std::unique_ptr<DBClientBase>(uri.connect(StringData(), errmsg, socketTimeout));
     uassert(40356, _name + ": connect failed " + uri.toString() + " : " + errmsg, c);

     return _finishCreate(uri.toString(), socketTimeout, c.release());
}