Thank you for pointing this out to me in #331 !
1.There is a matching test, /frontend/spec/mws/keyword.spec.js , to which we could add; 
    it('can tell users that only one database exists here', function () {
      mongo.keyword.show(shell, ['dbs']);
      expect(shell.insertResponseLine).toHaveBeenCalledWith("The web shell only allows one db.");
    });
2.Also I was wondering if this wording might be better? 
"The web shell only allows one db. Type 'db' to see which one you are using."