SystemTools: MakeDirectory() reports failure if path is file

If the path passed to MakeDirectory() pointed to an existing file (not
directory), it would return true indicating that the directory was
successfully created even though creation of the directory failed.
Now it returns true only when the existing path is a directory and
returns false otherwise.

Change-Id: Ieb3e6352d63868c824df35f19187a8d396f75296