If the log filepath points to an existing directory display an error saying this.

Right now you get something like this: "can't open [c:\temp\mongodblogs] for log file: errno:13 The data is invalid."

It would be much more helpful to have "errno:13 The data is invalid. Can't open [c:\temp\mongodblogs]. It cannot be used for a log file-path since it is a directory.