Fix org-format-latex: don't throw an error if dir exists.

Fix org-format-latex: don't throw an error if dir exists.

From: Alexandre Passos <alexandre.tp@gmail.com>

> I was editing an org document on a server earlier today, remotely
> using tramp, and continuously exporting it to html. When I added
> LaTeX, it exported once and then not anymore, failing because it
> couldn't create a directory anymore. So I found out that patching
> org-export-latex to pass a "t" parameter to org-make-directory fixes
> this, and it continues to work perfectly. This is the modified version
> of that function, if anyone else is interested in this constrained
> case. The only change I made was right under the "make sure directory
> exists" comment.